diff --git a/ErsatzTV.Application/ConcurrencyExtensions.cs b/ErsatzTV.Application/ConcurrencyExtensions.cs
index e98cdabd9..5d29c7d31 100644
--- a/ErsatzTV.Application/ConcurrencyExtensions.cs
+++ b/ErsatzTV.Application/ConcurrencyExtensions.cs
@@ -1,6 +1,7 @@
 using ErsatzTV.Core;
 using ErsatzTV.Core.Domain;
 using ErsatzTV.Core.Errors;
+using ErsatzTV.Infrastructure.Data;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.ChangeTracking;
 
@@ -70,6 +71,32 @@ public static class ConcurrencyExtensions
         }
     }
 
+    /// <summary>
+    ///     Like <see cref="SaveChangesForcingVersion" />, but additionally treats a unique / primary-key
+    ///     constraint violation as an idempotent no-op: returns <c>false</c> instead of throwing when the
+    ///     save fails because a concurrent request inserted a row we had membership-checked absent (the
+    ///     composite-PK race on <c>CollectionItem</c> — issue #308). A <c>false</c> means "the desired row
+    ///     already exists because a racing writer won; the winner ran the ETag rotation + fan-out, so skip
+    ///     ours." <c>true</c> means our own change committed. Every other <see cref="DbUpdateException" />
+    ///     (and the genuine deleted-row concurrency conflict rethrown by <see cref="SaveChangesForcingVersion" />)
+    ///     still propagates. The only insert these callers stage is the <c>CollectionItem</c> join row, so the
+    ///     sole unique/PK constraint that can fire here is that composite key.
+    /// </summary>
+    public static async Task<bool> TrySaveChangesForcingVersion(
+        this DbContext dbContext,
+        CancellationToken cancellationToken)
+    {
+        try
+        {
+            await dbContext.SaveChangesForcingVersion(cancellationToken);
+            return true;
+        }
+        catch (DbUpdateException ex) when (TvContext.IsUniqueConstraintViolation(ex))
+        {
+            return false;
+        }
+    }
+
     /// <summary>
     ///     Persist pending changes, mapping the EF optimistic-concurrency failure to
     ///     <see cref="PreconditionFailedError" /> (→ 412). When a versioned root carries an
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddArtistToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddArtistToCollectionHandler.cs
index df3c4c829..2a15b8349 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddArtistToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddArtistToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -55,7 +55,13 @@ public class AddArtistToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Artist.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddEpisodeToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddEpisodeToCollectionHandler.cs
index b46405fc7..223c94fa7 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddEpisodeToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddEpisodeToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -57,7 +57,13 @@ public class AddEpisodeToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Episode.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddImageToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddImageToCollectionHandler.cs
index 2eba28437..6a6b81656 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddImageToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddImageToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -54,7 +54,13 @@ public class AddImageToCollectionHandler : IRequestHandler<AddImageToCollection,
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Image.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddItemsToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddItemsToCollectionHandler.cs
index e325e8588..6c55fffd7 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddItemsToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddItemsToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -38,23 +38,52 @@ public class AddItemsToCollectionHandler :
         _searchChannel = searchChannel;
     }
 
+    // A duplicate-key race can roll back the whole batch (#308); recompute membership from a fresh
+    // context and retry with only the still-missing items. Bounded to avoid a livelock — the common
+    // no-collision path runs the loop body exactly once.
+    private const int MaxDuplicateRetries = 5;
+
     public async Task<Either<BaseError, Unit>> Handle(
         AddItemsToCollection request,
         CancellationToken cancellationToken)
     {
-        await using TvContext dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
-        Option<Collection> maybeCollection = await CollectionMustExist(dbContext, request, cancellationToken);
-        return await maybeCollection.Match(
-            Some: async collection =>
+        for (var attempt = 0; ; attempt++)
+        {
+            await using TvContext dbContext = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
+            Option<Collection> maybeCollection = await CollectionMustExist(dbContext, request, cancellationToken);
+
+            // true = terminal (nothing to add, or our batch committed); false = a duplicate-key race
+            // rolled the batch back, recompute membership and retry.
+            Either<BaseError, bool> attemptResult = await maybeCollection.Match(
+                Some: async collection =>
+                {
+                    Validation<BaseError, Collection> validation = await Validate(dbContext, request, collection, cancellationToken);
+                    return await validation.Apply(c => ApplyAddItemsRequest(dbContext, c, request, cancellationToken));
+                },
+                None: () => Task.FromResult<Either<BaseError, bool>>(
+                    new NotFoundError($"Collection {request.CollectionId} does not exist.")));
+
+            if (attemptResult.IsLeft)
             {
-                Validation<BaseError, Collection> validation = await Validate(dbContext, request, collection, cancellationToken);
-                return await validation.Apply(c => ApplyAddItemsRequest(dbContext, c, request, cancellationToken));
-            },
-            None: () => Task.FromResult<Either<BaseError, Unit>>(
-                new NotFoundError($"Collection {request.CollectionId} does not exist.")));
+                return attemptResult.Map(_ => Unit.Default);
+            }
+
+            bool committed = attemptResult.Match(Left: _ => false, Right: done => done);
+            if (committed)
+            {
+                return Unit.Default;
+            }
+
+            // A concurrent add inserted one+ of our items first; recompute against fresh membership.
+            if (attempt >= MaxDuplicateRetries)
+            {
+                return BaseError.New(
+                    "Concurrent modification while adding items to the collection; please retry.");
+            }
+        }
     }
 
-    private async Task<Unit> ApplyAddItemsRequest(
+    private async Task<bool> ApplyAddItemsRequest(
         TvContext dbContext,
         Collection collection,
         AddItemsToCollection request,
@@ -75,10 +104,10 @@ public class AddItemsToCollectionHandler :
         var toAddIds = allItems.Where(item => collection.MediaItems.All(mi => mi.Id != item)).ToList();
 
         // No-op when every requested item is already a member: don't rotate the ETag or fan out
-        // rebuilds for an idempotent re-add — #269.
+        // rebuilds for an idempotent re-add — #269. Terminal success (no retry).
         if (toAddIds.Count == 0)
         {
-            return Unit.Default;
+            return true;
         }
 
         List<MediaItem> toAdd = await dbContext.MediaItems
@@ -91,7 +120,15 @@ public class AddItemsToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a).
         collection.Version++;
-        await dbContext.SaveChangesForcingVersion(cancellationToken);
+
+        // A concurrent add of an overlapping item won the composite-PK race and rolled back this whole
+        // batch. Unlike the single-item handlers (idempotent no-op), a bulk add must NOT drop the items
+        // that did NOT collide — signal the caller to recompute membership and retry the still-missing
+        // ones. #308
+        if (!await dbContext.TrySaveChangesForcingVersion(cancellationToken))
+        {
+            return false;
+        }
 
         // post-commit side effect runs on CancellationToken.None so a late request cancellation
         // can't abort it after the commit landed (#254)
@@ -104,7 +141,7 @@ public class AddItemsToCollectionHandler :
             await _channel.WriteAsync(new BuildPlayout(playoutId, PlayoutBuildMode.Refresh), CancellationToken.None);
         }
 
-        return Unit.Default;
+        return true;
     }
 
     private async Task<Validation<BaseError, Collection>> Validate(
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddMediaItemToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddMediaItemToCollectionHandler.cs
index 195e6cff9..9949d124d 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddMediaItemToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddMediaItemToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -55,7 +55,13 @@ public class AddMediaItemToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.MediaItem.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddMovieToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddMovieToCollectionHandler.cs
index 603d4d49a..54290928e 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddMovieToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddMovieToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -55,7 +55,13 @@ public class AddMovieToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Movie.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddMusicVideoToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddMusicVideoToCollectionHandler.cs
index 39b2b21f4..29bc706fe 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddMusicVideoToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddMusicVideoToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -57,7 +57,13 @@ public class AddMusicVideoToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.MusicVideo.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddOtherVideoToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddOtherVideoToCollectionHandler.cs
index c76fcfa48..d54cdddab 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddOtherVideoToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddOtherVideoToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -57,7 +57,13 @@ public class AddOtherVideoToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.OtherVideo.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddSeasonToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddSeasonToCollectionHandler.cs
index 4613010eb..b3386a9bc 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddSeasonToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddSeasonToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -55,7 +55,13 @@ public class AddSeasonToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Season.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddShowToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddShowToCollectionHandler.cs
index f5bbc9fa0..e1fedb8dd 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddShowToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddShowToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -55,7 +55,13 @@ public class AddShowToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Show.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Application/MediaCollections/Commands/AddSongToCollectionHandler.cs b/ErsatzTV.Application/MediaCollections/Commands/AddSongToCollectionHandler.cs
index aea054ed2..084681c99 100644
--- a/ErsatzTV.Application/MediaCollections/Commands/AddSongToCollectionHandler.cs
+++ b/ErsatzTV.Application/MediaCollections/Commands/AddSongToCollectionHandler.cs
@@ -1,4 +1,4 @@
-using System.Threading.Channels;
+using System.Threading.Channels;
 using ErsatzTV.Application.Playouts;
 using ErsatzTV.Application.Search;
 using ErsatzTV.Core;
@@ -55,7 +55,13 @@ public class AddSongToCollectionHandler :
         // force-write past a concurrent replace-all Version bump — this add takes no If-Match, so a
         // benign race must not 500 (#253/#269 §7a). Post-commit enqueues run on CancellationToken.None.
         parameters.Collection.Version++;
-        await dbContext.SaveChangesForcingVersion(CancellationToken.None);
+        if (!await dbContext.TrySaveChangesForcingVersion(CancellationToken.None))
+        {
+            // A concurrent add of this same item won the composite-PK race and already inserted the row,
+            // rotated the collection ETag, and fanned out the rebuild — so this is now an idempotent
+            // no-op. Skip our reindex/rebuild fan-out (the winner already did it). #308
+            return Unit.Default;
+        }
 
         await _searchChannel.WriteAsync(new ReindexMediaItems([parameters.Song.Id]), CancellationToken.None);
 
diff --git a/ErsatzTV.Infrastructure.MySql/Data/MySqlErrorClassifier.cs b/ErsatzTV.Infrastructure.MySql/Data/MySqlErrorClassifier.cs
new file mode 100644
index 000000000..c22d85590
--- /dev/null
+++ b/ErsatzTV.Infrastructure.MySql/Data/MySqlErrorClassifier.cs
@@ -0,0 +1,17 @@
+using Microsoft.EntityFrameworkCore;
+using MySqlConnector;
+
+namespace ErsatzTV.Infrastructure.MySql.Data;
+
+/// <summary>
+///     Classifies MySQL provider exceptions wrapped by EF Core. Wired to
+///     <see cref="ErsatzTV.Infrastructure.Data.TvContext.IsUniqueConstraintViolation" /> at startup.
+/// </summary>
+public static class MySqlErrorClassifier
+{
+    // ER_DUP_ENTRY (1062): a duplicate value for a PRIMARY KEY or UNIQUE index.
+    private const int ErDupEntry = 1062;
+
+    public static bool IsUniqueConstraintViolation(DbUpdateException ex) =>
+        ex.InnerException is MySqlException { Number: ErDupEntry };
+}
diff --git a/ErsatzTV.Infrastructure.Sqlite/Data/SqliteErrorClassifier.cs b/ErsatzTV.Infrastructure.Sqlite/Data/SqliteErrorClassifier.cs
new file mode 100644
index 000000000..2ad1a9534
--- /dev/null
+++ b/ErsatzTV.Infrastructure.Sqlite/Data/SqliteErrorClassifier.cs
@@ -0,0 +1,22 @@
+using Microsoft.Data.Sqlite;
+using Microsoft.EntityFrameworkCore;
+
+namespace ErsatzTV.Infrastructure.Sqlite.Data;
+
+/// <summary>
+///     Classifies SQLite provider exceptions wrapped by EF Core. Wired to
+///     <see cref="ErsatzTV.Infrastructure.Data.TvContext.IsUniqueConstraintViolation" /> at startup.
+/// </summary>
+public static class SqliteErrorClassifier
+{
+    // SQLITE_CONSTRAINT (primary result code 19), narrowed to the two extended codes that mean a
+    // duplicate key: SQLITE_CONSTRAINT_PRIMARYKEY (1555) and SQLITE_CONSTRAINT_UNIQUE (2067). Other
+    // constraint failures (FK 787, NOT NULL 1299, CHECK 275) are deliberately NOT treated as duplicates.
+    private const int SqliteConstraint = 19;
+    private const int SqliteConstraintPrimaryKey = 1555;
+    private const int SqliteConstraintUnique = 2067;
+
+    public static bool IsUniqueConstraintViolation(DbUpdateException ex) =>
+        ex.InnerException is SqliteException { SqliteErrorCode: SqliteConstraint } inner
+        && inner.SqliteExtendedErrorCode is SqliteConstraintPrimaryKey or SqliteConstraintUnique;
+}
diff --git a/ErsatzTV.Infrastructure/Data/TvContext.cs b/ErsatzTV.Infrastructure/Data/TvContext.cs
index b226b4016..2e014ebc9 100644
--- a/ErsatzTV.Infrastructure/Data/TvContext.cs
+++ b/ErsatzTV.Infrastructure/Data/TvContext.cs
@@ -26,6 +26,16 @@ public class TvContext : DbContext
     public static string CaseInsensitiveCollation { get; set; } = "NOCASE";
     public static bool IsSqlite { get; set; }
 
+    /// <summary>
+    ///     Recognizes a provider-specific unique / primary-key constraint violation surfaced as a
+    ///     <see cref="DbUpdateException" /> (SQLite error 19 / MySQL 1062). Set at startup by the active
+    ///     provider's wiring, mirroring <see cref="IsSqlite" /> / <see cref="LastInsertedRowId" />. Used to
+    ///     turn a concurrent duplicate insert (a row we membership-checked absent, inserted first by a racing
+    ///     request) into an idempotent no-op instead of a 500 — see <c>ConcurrencyExtensions</c> and #308.
+    ///     Defaults to a conservative "no" so an unwired provider never silently swallows a save failure.
+    /// </summary>
+    public static Func<DbUpdateException, bool> IsUniqueConstraintViolation { get; set; } = static _ => false;
+
     public IDbConnection Connection => Database.GetDbConnection();
 
     public DbSet<ConfigElement> ConfigElements { get; set; }
diff --git a/ErsatzTV.Tests/Application/MediaCollections/AddToCollectionIdempotencyConcurrencyTests.cs b/ErsatzTV.Tests/Application/MediaCollections/AddToCollectionIdempotencyConcurrencyTests.cs
new file mode 100644
index 000000000..93eb48c87
--- /dev/null
+++ b/ErsatzTV.Tests/Application/MediaCollections/AddToCollectionIdempotencyConcurrencyTests.cs
@@ -0,0 +1,256 @@
+using System.Threading.Channels;
+using ErsatzTV.Application;
+using ErsatzTV.Application.MediaCollections;
+using ErsatzTV.Application.Search;
+using ErsatzTV.Core;
+using ErsatzTV.Core.Domain;
+using ErsatzTV.Core.Interfaces.Repositories;
+using ErsatzTV.Infrastructure.Data;
+using ErsatzTV.Infrastructure.Sqlite.Data;
+using ErsatzTV.Tests.Support;
+using LanguageExt;
+using Microsoft.Data.Sqlite;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Diagnostics;
+using NSubstitute;
+using NUnit.Framework;
+using Shouldly;
+using Unit = LanguageExt.Unit;
+
+namespace ErsatzTV.Tests.Application.MediaCollections;
+
+/// <summary>
+///     #308: two concurrent adds of the same item both membership-check it absent, both insert the same
+///     <c>CollectionItem</c> composite key, and the loser used to 500 on the unhandled
+///     <see cref="DbUpdateException" /> (SQLite 19 / MySQL 1062) that <c>SaveChangesForcingVersion</c>
+///     does not catch. The fix (<c>TrySaveChangesForcingVersion</c>) makes the loser an idempotent no-op
+///     for the single-item handlers, and for the bulk handler retries against fresh membership so the
+///     non-colliding items are not dropped.
+/// </summary>
+[TestFixture]
+public class AddToCollectionIdempotencyConcurrencyTests
+{
+    private InMemoryTvContext _db = null!;
+
+    [SetUp]
+    public async Task SetUp() => _db = await InMemoryTvContext.CreateAsync();
+
+    [TearDown]
+    public async Task TearDown() => await _db.DisposeAsync();
+
+    private static async Task SeedCollectionWithMovies(Func<TvContext> createContext, int collectionId, params int[] movieIds)
+    {
+        await using TvContext context = createContext();
+
+        var library = new LocalLibrary
+        {
+            Id = 1, Name = "Library", MediaKind = LibraryMediaKind.Movies, Paths = []
+        };
+        var path = new LibraryPath { Id = 1, Path = "/media", Library = library, LibraryFolders = [], MediaItems = [] };
+        library.Paths.Add(path);
+
+        foreach (int movieId in movieIds)
+        {
+            context.Movies.Add(new Movie
+            {
+                Id = movieId,
+                LibraryPath = path,
+                Collections = [],
+                CollectionItems = [],
+                TraktListItems = [],
+                MovieMetadata =
+                [
+                    new MovieMetadata
+                    {
+                        Title = $"Movie {movieId}", SortTitle = $"Movie {movieId}", Artwork = [], Genres = [],
+                        Tags = [], Studios = [], Actors = [], Guids = [], Subtitles = [], Directors = [], Writers = []
+                    }
+                ],
+                MediaVersions = [new MediaVersion { Duration = TimeSpan.FromMinutes(30) }]
+            });
+        }
+
+        context.Collections.Add(new Collection
+        {
+            Id = collectionId, Name = "Manual", Version = 1, MediaItems = [], CollectionItems = []
+        });
+
+        await context.SaveChangesAsync();
+    }
+
+    private static async Task<int> CollectionItemCount(Func<TvContext> createContext, int collectionId, int mediaItemId)
+    {
+        await using TvContext context = createContext();
+        return await context.CollectionItems
+            .CountAsync(ci => ci.CollectionId == collectionId && ci.MediaItemId == mediaItemId);
+    }
+
+    private static (IMediaCollectionRepository repo, Channel<IBackgroundServiceRequest> worker,
+        ChannelWriter<ISearchIndexBackgroundServiceRequest> search) Deps()
+    {
+        IMediaCollectionRepository repo = Substitute.For<IMediaCollectionRepository>();
+        repo.PlayoutIdsUsingCollection(Arg.Any<int>()).Returns([1]);
+        Channel<IBackgroundServiceRequest> worker =
+            System.Threading.Channels.Channel.CreateUnbounded<IBackgroundServiceRequest>();
+        ChannelWriter<ISearchIndexBackgroundServiceRequest> search =
+            System.Threading.Channels.Channel.CreateUnbounded<ISearchIndexBackgroundServiceRequest>().Writer;
+        return (repo, worker, search);
+    }
+
+    // Simulates the concurrent "winner": exactly once, on a SEPARATE connection, insert the same composite
+    // key and commit — AFTER the intercepted context has read (stale) membership but BEFORE its own INSERT
+    // runs. This is what interposes the race deterministically.
+    private sealed class InsertConflictingItemOnce(SharedCacheTvContext db, int collectionId, int mediaItemId)
+        : SaveChangesInterceptor
+    {
+        private int _fired;
+
+        public override async ValueTask<InterceptionResult<int>> SavingChangesAsync(
+            DbContextEventData eventData,
+            InterceptionResult<int> result,
+            CancellationToken cancellationToken = default)
+        {
+            if (Interlocked.Exchange(ref _fired, 1) == 0)
+            {
+                await using SqliteConnection connection = db.OpenConnection();
+                await using SqliteCommand command = connection.CreateCommand();
+                command.CommandText =
+                    "INSERT INTO \"CollectionItem\" (\"CollectionId\", \"MediaItemId\", \"CustomIndex\") " +
+                    $"VALUES ({collectionId}, {mediaItemId}, NULL)";
+                await command.ExecuteNonQueryAsync(cancellationToken);
+            }
+
+            return result;
+        }
+    }
+
+    // ----- Non-vacuous anchor + extension-level unit tests (single shared connection) -----
+
+    // Proves the race really does throw a DbUpdateException the classifier recognizes — i.e. the pre-fix
+    // 500 was real and the fix's catch filter is not a no-op. This is the negative control anchoring the
+    // extension tests below.
+    [Test]
+    public async Task Duplicate_CollectionItem_Insert_Throws_A_Classified_UniqueViolation()
+    {
+        await SeedCollectionWithMovies(_db.CreateContext, 1, 10);
+
+        await using TvContext stale = _db.CreateContext();
+        Collection tracked = await stale.Collections.Include(c => c.MediaItems).SingleAsync(c => c.Id == 1);
+        Movie movie = await stale.Movies.SingleAsync(m => m.Id == 10);
+
+        // A racer inserts the (1, 10) CollectionItem and commits, then the stale context tries the same key.
+        await InsertRow(_db.CreateContext, 1, 10);
+        tracked.MediaItems.Add(movie);
+
+        DbUpdateException ex = await Should.ThrowAsync<DbUpdateException>(() => stale.SaveChangesAsync());
+        SqliteErrorClassifier.IsUniqueConstraintViolation(ex).ShouldBeTrue();
+
+        // And the classifier does not blanket-return true for an unrelated failure.
+        SqliteErrorClassifier.IsUniqueConstraintViolation(
+            new DbUpdateException("nope", new InvalidOperationException())).ShouldBeFalse();
+    }
+
+    [Test]
+    public async Task TrySaveChangesForcingVersion_Swallows_The_Duplicate_And_Returns_False()
+    {
+        await SeedCollectionWithMovies(_db.CreateContext, 1, 10);
+
+        await using TvContext stale = _db.CreateContext();
+        Collection tracked = await stale.Collections.Include(c => c.MediaItems).SingleAsync(c => c.Id == 1);
+        Movie movie = await stale.Movies.SingleAsync(m => m.Id == 10);
+
+        await InsertRow(_db.CreateContext, 1, 10);
+        tracked.MediaItems.Add(movie);
+        tracked.Version++;
+
+        bool committed = await stale.TrySaveChangesForcingVersion(CancellationToken.None);
+
+        committed.ShouldBeFalse(); // the loser's save was swallowed, not thrown
+        (await CollectionItemCount(_db.CreateContext, 1, 10)).ShouldBe(1); // exactly one row, no corruption
+    }
+
+    [Test]
+    public async Task TrySaveChangesForcingVersion_Rethrows_When_The_Provider_Does_Not_Classify_It()
+    {
+        // Guard against an over-broad catch: with the classifier saying "not a unique violation", the
+        // duplicate must still propagate rather than be silently swallowed.
+        Func<DbUpdateException, bool> original = TvContext.IsUniqueConstraintViolation;
+        try
+        {
+            await SeedCollectionWithMovies(_db.CreateContext, 1, 10);
+
+            await using TvContext stale = _db.CreateContext();
+            Collection tracked = await stale.Collections.Include(c => c.MediaItems).SingleAsync(c => c.Id == 1);
+            Movie movie = await stale.Movies.SingleAsync(m => m.Id == 10);
+
+            await InsertRow(_db.CreateContext, 1, 10);
+            tracked.MediaItems.Add(movie);
+            tracked.Version++;
+
+            TvContext.IsUniqueConstraintViolation = _ => false;
+            await Should.ThrowAsync<DbUpdateException>(() => stale.TrySaveChangesForcingVersion(CancellationToken.None));
+        }
+        finally
+        {
+            TvContext.IsUniqueConstraintViolation = original;
+        }
+    }
+
+    // ----- End-to-end handler tests over a real cross-connection race (shared cache) -----
+
+    [Test]
+    public async Task SingleItem_Handler_Losing_The_Cross_Connection_Race_Is_A_NoOp_Not_A_500()
+    {
+        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv308-single");
+        await SeedCollectionWithMovies(db.CreateContext, 1, 10);
+
+        (IMediaCollectionRepository repo, Channel<IBackgroundServiceRequest> worker,
+            ChannelWriter<ISearchIndexBackgroundServiceRequest> search) = Deps();
+
+        // The interceptor inserts (1, 10) on another connection during this handler's save.
+        var racer = new InsertConflictingItemOnce(db, 1, 10);
+        var handler = new AddMovieToCollectionHandler(db.Factory(racer), repo, worker.Writer, search);
+
+        Either<BaseError, Unit> result = await handler.Handle(new AddMovieToCollection(1, 10), CancellationToken.None);
+
+        result.IsRight.ShouldBeTrue(); // no 500
+        (await CollectionItemCount(db.CreateContext, 1, 10)).ShouldBe(1); // one row, no duplicate
+        worker.Reader.Count.ShouldBe(0); // the loser does not fan out a rebuild (the winner already did)
+    }
+
+    [Test]
+    public async Task Bulk_Handler_Losing_The_Cross_Connection_Race_Retries_And_Keeps_NonColliding_Items()
+    {
+        await using SharedCacheTvContext db = await SharedCacheTvContext.CreateAsync("etv308-bulk");
+        await SeedCollectionWithMovies(db.CreateContext, 1, 10, 11);
+
+        (IMediaCollectionRepository repo, Channel<IBackgroundServiceRequest> worker,
+            ChannelWriter<ISearchIndexBackgroundServiceRequest> search) = Deps();
+
+        var movieRepo = Substitute.For<IMovieRepository>();
+        movieRepo.AllMoviesExist(Arg.Any<List<int>>()).Returns(true);
+        var tvRepo = Substitute.For<ITelevisionRepository>();
+        tvRepo.AllShowsExist(Arg.Any<List<int>>()).Returns(true);
+        tvRepo.AllSeasonsExist(Arg.Any<List<int>>()).Returns(true);
+        tvRepo.AllEpisodesExist(Arg.Any<List<int>>()).Returns(true);
+
+        // Item 10 is inserted by the racer during the first attempt's save, colliding and rolling back the
+        // whole [10, 11] batch; the retry must recompute membership and still add 11.
+        var racer = new InsertConflictingItemOnce(db, 1, 10);
+        var handler = new AddItemsToCollectionHandler(db.Factory(racer), repo, movieRepo, tvRepo, worker.Writer, search);
+
+        var request = new AddItemsToCollection(1, [10, 11], [], [], [], [], [], [], [], [], []);
+        Either<BaseError, Unit> result = await handler.Handle(request, CancellationToken.None);
+
+        result.IsRight.ShouldBeTrue();
+        (await CollectionItemCount(db.CreateContext, 1, 10)).ShouldBe(1); // the racer's row — not dropped/duped
+        (await CollectionItemCount(db.CreateContext, 1, 11)).ShouldBe(1); // NOT dropped by the rolled-back attempt
+    }
+
+    private static async Task InsertRow(Func<TvContext> createContext, int collectionId, int mediaItemId)
+    {
+        await using TvContext context = createContext();
+        context.Add(new CollectionItem { CollectionId = collectionId, MediaItemId = mediaItemId, CustomIndex = null });
+        await context.SaveChangesAsync();
+    }
+}
diff --git a/ErsatzTV.Tests/Support/InMemoryTvContext.cs b/ErsatzTV.Tests/Support/InMemoryTvContext.cs
index 93def6448..5dcb34160 100644
--- a/ErsatzTV.Tests/Support/InMemoryTvContext.cs
+++ b/ErsatzTV.Tests/Support/InMemoryTvContext.cs
@@ -1,5 +1,6 @@
 using ErsatzTV.Infrastructure;
 using ErsatzTV.Infrastructure.Data;
+using ErsatzTV.Infrastructure.Sqlite.Data;
 using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging.Abstractions;
@@ -29,6 +30,7 @@ public sealed class InMemoryTvContext : IAsyncDisposable
     public static async Task<InMemoryTvContext> CreateAsync()
     {
         TvContext.IsSqlite = true;
+        TvContext.IsUniqueConstraintViolation = SqliteErrorClassifier.IsUniqueConstraintViolation;
 
         var connection = new SqliteConnection("Data Source=:memory:;Foreign Keys=False");
         await connection.OpenAsync();
diff --git a/ErsatzTV.Tests/Support/SharedCacheTvContext.cs b/ErsatzTV.Tests/Support/SharedCacheTvContext.cs
new file mode 100644
index 000000000..970a339bd
--- /dev/null
+++ b/ErsatzTV.Tests/Support/SharedCacheTvContext.cs
@@ -0,0 +1,89 @@
+using ErsatzTV.Infrastructure;
+using ErsatzTV.Infrastructure.Data;
+using ErsatzTV.Infrastructure.Sqlite.Data;
+using Microsoft.Data.Sqlite;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Diagnostics;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace ErsatzTV.Tests.Support;
+
+/// <summary>
+///     A multi-connection in-memory SQLite harness (shared cache) so several <see cref="TvContext" />
+///     instances on DISTINCT connections hit the same database and real unique-constraint enforcement
+///     applies across connections. Unlike <see cref="InMemoryTvContext" /> (a single shared connection),
+///     this can reproduce a genuine cross-connection write race — needed to exercise the #308 composite-PK
+///     collision through the real handler. A keep-alive connection holds the shared in-memory DB open for
+///     the harness lifetime.
+/// </summary>
+public sealed class SharedCacheTvContext : IAsyncDisposable
+{
+    private readonly string _connectionString;
+    private readonly SqliteConnection _keepAlive;
+
+    private SharedCacheTvContext(string connectionString, SqliteConnection keepAlive)
+    {
+        _connectionString = connectionString;
+        _keepAlive = keepAlive;
+    }
+
+    public static async Task<SharedCacheTvContext> CreateAsync(string name)
+    {
+        TvContext.IsSqlite = true;
+        TvContext.IsUniqueConstraintViolation = SqliteErrorClassifier.IsUniqueConstraintViolation;
+
+        // Foreign keys OFF (connection-string keyword, inherited by every connection) so partial object
+        // graphs can be seeded without satisfying every FK — matches InMemoryTvContext. The composite-PK /
+        // UNIQUE constraint under test is enforced regardless of this pragma.
+        var connectionString = $"Data Source={name};Mode=Memory;Cache=Shared;Foreign Keys=False";
+        var keepAlive = new SqliteConnection(connectionString);
+        await keepAlive.OpenAsync();
+
+        await using (TvContext context = Create(BuildOptions(connectionString, null)))
+        {
+            await context.Database.EnsureCreatedAsync();
+        }
+
+        return new SharedCacheTvContext(connectionString, keepAlive);
+    }
+
+    /// <summary>A factory whose contexts each open their own connection to the shared DB.</summary>
+    public IDbContextFactory<TvContext> Factory(IInterceptor? interceptor = null) =>
+        new PerConnectionFactory(_connectionString, interceptor);
+
+    public TvContext CreateContext() => Create(BuildOptions(_connectionString, null));
+
+    /// <summary>A freshly opened raw connection to the shared DB (used to stage a concurrent writer).</summary>
+    public SqliteConnection OpenConnection()
+    {
+        var connection = new SqliteConnection(_connectionString);
+        connection.Open();
+        return connection;
+    }
+
+    public async ValueTask DisposeAsync() => await _keepAlive.DisposeAsync();
+
+    private static DbContextOptions<TvContext> BuildOptions(string connectionString, IInterceptor? interceptor)
+    {
+        DbContextOptionsBuilder<TvContext> builder = new DbContextOptionsBuilder<TvContext>()
+            .UseSqlite(connectionString);
+        if (interceptor is not null)
+        {
+            builder.AddInterceptors(interceptor);
+        }
+
+        return builder.Options;
+    }
+
+    private static TvContext Create(DbContextOptions<TvContext> options) =>
+        new(
+            options,
+            NullLoggerFactory.Instance,
+            new SlowQueryInterceptor(NullLogger<SlowQueryInterceptor>.Instance));
+
+    private sealed class PerConnectionFactory(string connectionString, IInterceptor? interceptor)
+        : IDbContextFactory<TvContext>
+    {
+        public TvContext CreateDbContext() => Create(BuildOptions(connectionString, interceptor));
+    }
+}
diff --git a/ErsatzTV/Startup.cs b/ErsatzTV/Startup.cs
index dd5aebf21..c147544ca 100644
--- a/ErsatzTV/Startup.cs
+++ b/ErsatzTV/Startup.cs
@@ -71,6 +71,7 @@ using ErsatzTV.Infrastructure.Locking;
 using ErsatzTV.Infrastructure.Metadata;
 using ErsatzTV.Infrastructure.Plex;
 using ErsatzTV.Infrastructure.Runtime;
+using ErsatzTV.Infrastructure.MySql.Data;
 using ErsatzTV.Infrastructure.Scheduling;
 using ErsatzTV.Infrastructure.Scripting;
 using ErsatzTV.Infrastructure.Search;
@@ -647,6 +648,7 @@ public class Startup
 
             TvContext.LastInsertedRowId = "last_insert_rowid()";
             TvContext.CaseInsensitiveCollation = "NOCASE";
+            TvContext.IsUniqueConstraintViolation = SqliteErrorClassifier.IsUniqueConstraintViolation;
 
             SqlMapper.AddTypeHandler(new DateTimeOffsetHandler());
             SqlMapper.AddTypeHandler(new GuidHandler());
@@ -657,6 +659,7 @@ public class Startup
         {
             TvContext.LastInsertedRowId = "last_insert_id()";
             TvContext.CaseInsensitiveCollation = "utf8mb4_general_ci";
+            TvContext.IsUniqueConstraintViolation = MySqlErrorClassifier.IsUniqueConstraintViolation;
         }
 
         Log.Logger.Information("Transcode folder is {Folder}", FileSystemLayout.TranscodeFolder);
diff --git a/docs/api-conventions.md b/docs/api-conventions.md
index 864d7e9d7..45855c951 100644
--- a/docs/api-conventions.md
+++ b/docs/api-conventions.md
@@ -577,11 +577,25 @@ rotate the editor ETag). **No-op idempotence (the trap):** these handlers gate t
 fan-out on `SaveChanges() > 0`; an *unconditional* bump makes that gate always-true, so an idempotent re-add /
 same-value re-submit would fire spurious rebuilds. Each therefore short-circuits a genuine no-op **before** the
 bump — the Add handlers by an explicit membership check (also fixing the latent duplicate-`CollectionItem`
-insert on a *sequential* re-add; two *concurrent* adds of the same item can still both pass the check and the
-loser 500s on the composite-PK violation — a narrow, pre-existing race, tracked as #308), the scalar
-writers by `ChangeTracker.HasChanges()` — so a no-op neither bumps nor rebuilds. This is an
-invalidation-completeness refinement; the primary endpoints' own bump+guard already covered the two-tab
-lost-update the contract targets.
+insert on a *sequential* re-add), the scalar writers by `ChangeTracker.HasChanges()` — so a no-op neither
+bumps nor rebuilds. This is an invalidation-completeness refinement; the primary endpoints' own bump+guard
+already covered the two-tab lost-update the contract targets.
+
+**Idempotent insert under concurrency (#308).** The membership pre-check is not atomic with the insert, so
+two *concurrent* adds of the same item both observe it absent, both stage the `CollectionItem` composite key,
+and the loser's `SaveChangesForcingVersion` throws a unique/PK-violation `DbUpdateException` (SQLite error 19 /
+MySQL 1062) it does not catch → a 500. The `Add*ToCollection` family therefore saves through
+**`ConcurrencyExtensions.TrySaveChangesForcingVersion`** (a `bool`-returning sibling of `SaveChangesForcingVersion`)
+which catches *only* that classified violation and returns `false`. The single-item handlers treat `false` as an
+idempotent **no-op** (the racing winner already inserted the row, rotated the ETag, and fanned out the rebuild);
+the bulk `AddItemsToCollection` handler instead **retries** on a fresh context against recomputed membership so
+the non-colliding items in the batch are not dropped (bounded loop; the common no-collision path runs once).
+The provider-specific classifier is wired the same way as the other provider statics on `TvContext` — a settable
+`TvContext.IsUniqueConstraintViolation` delegate pointed at `SqliteErrorClassifier` / `MySqlErrorClassifier`
+(`ErsatzTV.Infrastructure.Sqlite/MySql.Data`) from `Startup.cs`, defaulting to a conservative "no" so an unwired
+provider never silently swallows a save failure. `Add*ToPlaylist` is **not** affected: `PlaylistItem` has its own
+identity PK and no unique index on `(PlaylistId, MediaItemId)` — a playlist may legitimately contain the same item
+more than once, so there is no constraint to violate.
 
 ## 7b. Post-commit side effects run on `CancellationToken.None`
 
diff --git a/docs/decisions/optimistic-concurrency.md b/docs/decisions/optimistic-concurrency.md
index 3a8484e8c..4a54a1532 100644
--- a/docs/decisions/optimistic-concurrency.md
+++ b/docs/decisions/optimistic-concurrency.md
@@ -17,6 +17,7 @@ cross-editor ETag rotation). Refs #197.
 - [2026-07-12 (#269 — non-If-Match root writers force-write past a concurrent Version bump)](#2026-07-12-269--non-if-match-root-writers-force-write-past-a-concurrent-version-bump)
 - [2026-07-12 — Cross-editor ETag rotation completed for Collection/Playout config siblings (#269)](#2026-07-12--cross-editor-etag-rotation-completed-for-collectionplayout-config-siblings-269)
 - [2026-07-12 — If-Match evaluates per RFC 7232: valid-but-non-matching → 412, only grammar violations → 400 (#265)](#2026-07-12--if-match-evaluates-per-rfc-7232-valid-but-non-matching--412-only-grammar-violations--400-265)
+- [2026-07-18 — Concurrent same-item add is idempotent, not a 500: catch the unique-violation per provider (#308)](#2026-07-18--concurrent-same-item-add-is-idempotent-not-a-500-catch-the-unique-violation-per-provider-308)
 
 ---
 
@@ -254,3 +255,36 @@ worried about was already correct (each handler loads/validates → 404 before `
 first-party SPA only ever echoes the single canonical strong tag we emit, so no shipped client changes behavior;
 the change only makes a hand-written/tooling `If-Match` get the RFC-correct status. Docs: `api-conventions.md`
 §7a. Refs #265 #253 #197.
+
+## 2026-07-18 — Concurrent same-item add is idempotent, not a 500: catch the unique-violation per provider (#308)
+
+**Decision.** The `Add*ToCollection` family's membership pre-check (#269) is not atomic with the insert, so two
+*concurrent* adds of the same item both observe it absent and both stage the `CollectionItem` composite key; the
+loser's `SaveChangesForcingVersion` threw a unique/PK-violation `DbUpdateException` (SQLite error 19 / MySQL 1062)
+it did not catch → **500**. We now treat that loss as an **idempotent no-op**, not an error: the desired end state
+(the item is a member) already holds because the racing winner inserted it, rotated the ETag, and fanned out the
+rebuild.
+
+**Mechanism.** A `bool`-returning sibling `ConcurrencyExtensions.TrySaveChangesForcingVersion` wraps
+`SaveChangesForcingVersion` and catches *only* a classified unique/PK violation, returning `false`. The 10
+single-item handlers return `Unit.Default` on `false` (skip the reindex/rebuild fan-out — the winner did it). The
+bulk `AddItemsToCollection` handler cannot no-op — that would silently drop the non-colliding items when a batch
+partially overlaps a concurrent add — so it **retries** on a fresh context against recomputed membership (bounded
+loop; the common no-collision path runs once).
+
+**Provider seam.** Detection is provider-specific but the Application layer must not reference the provider
+packages, so it follows the existing `TvContext` static-provider-config idiom (`IsSqlite`, `LastInsertedRowId`): a
+settable `TvContext.IsUniqueConstraintViolation` delegate, pointed at `SqliteErrorClassifier` (extended codes 1555
+PK / 2067 UNIQUE) or `MySqlErrorClassifier` (`Number == 1062`) from `Startup.cs`, defaulting to a conservative
+"no" so an unwired provider never silently swallows a save failure. Chosen over DI to avoid threading a new
+service through 11 handlers, and because the provider discriminator already lives as a `TvContext` static.
+
+**Scope boundary.** `Add*ToPlaylist` is deliberately **untouched**: `PlaylistItem` has its own identity PK and no
+unique index on `(PlaylistId, MediaItemId)` — a playlist may legitimately contain the same item more than once, so
+there is no constraint to violate.
+
+**Tests.** A negative-control anchor proves the race genuinely throws a classified `DbUpdateException`; the fix's
+end-to-end handler tests reproduce a *real cross-connection* race via a shared-cache SQLite harness + a
+`SavingChanges` interceptor that inserts the conflicting row on another connection mid-save (the single-connection
+in-memory fixture cannot). Every fix-dependent test was verified to fail with the catch disabled. Mechanics:
+`api-conventions.md` §7a ("Idempotent insert under concurrency"). Refs #308 #269 #253.