timothy 2f2bcca681 fix(500): dedup incoming metadata collections so a duplicate name inserts once ...

The remove-stale + add-new reconcile idiom materializes its add set with
.ToList() BEFORE the loop mutates the existing collection, so the add filter
(`incoming.All(x2 => x2.Name != x.Name)`) is evaluated against a snapshot. Two
identically-named incoming entries whose name is not yet on the existing item
therefore BOTH passed the filter and BOTH inserted — a duplicate row.

Deduplicate the incoming set on the same key the filter compares (Name; Guid
for Guids), in both copies of the idiom:

- PlexMovieLibraryScanner.UpdateMetadata (the original) — genres, studios,
  actors, directors, writers, guids, tags.
- JellyfinMusicVideoLibraryScanner.Reconcile{Genres,Tags,Studios,Artists}
  (added in #497, mirrors the Plex pattern verbatim).

Plex ACTORS are the exception and get an artwork-preferring dedup hoisted out
and shared with the remove filter, because that filter is keyed on
(Name, artwork-presence) — it is the mechanism that drops an artwork-less actor
so the add loop can re-add it WITH artwork. A bare DistinctBy(a => a.Name)
there keeps the FIRST duplicate, so Plex listing the artwork-less copy first
discarded the artwork; worse, the remove filter would still see the
artwork-less duplicate, making its upgrade clause false, so the stale row was
never removed and the artwork never arrived on ANY later scan either. Actor
also carries Role/Order, which first-wins would silently drop too. Caught by
the cold review of the first version of this commit.

For the Jellyfin scanner the dedup sits at the incoming-list declaration, which
also covers the remove filter — safe because all four of those filters only ask
"is this name present at all", an answer duplicates cannot change.

Tests: duplicate-collapse for both paths, the two Actors cases above, and a
POSITIVE CONTROL proving distinct entries are still all added and stale ones
still removed (without it, a mis-keyed dedup that collapsed genuinely different
entries would pass every other assertion). Each proven non-vacuous.
The Plex tests drive the protected UpdateMetadata through a minimal test-only
subclass, as MediaServerMovieLibraryScannerTests already does.

Low likelihood in practice (a media server emitting two identically-named
genres for one item is unusual); this is defensive, with no observed occurrence.

The same idiom is copied into 8 further scanners/repositories that this change
deliberately does not touch (the issue scoped it to two paths) — filed as #600
so the class of bug is tracked rather than silently left in the majority of its
instances. The dedup rule is recorded under scan.musicvideo-reconciliation.

fixes #500

2026-07-25 13:04:38 +02:00

..

PlexMovieLibraryScannerTests.cs

fix(500): dedup incoming metadata collections so a duplicate name inserts once

2026-07-25 13:04:38 +02:00