fix(plex): S5a review round 2 — dup-HU guard + tests for dup-guid & guid-keep

- Correctness: _sync_overlay now first-wins on a guid (a `seen` set) so a duplicate HU entry
  sharing a guid can't overwrite the applied Hungarian metadata or double-count `overlaid`.
- Test-coverage: cover the earlier dup-guid fix (both editions of a shared-guid pair get *_hu,
  counted once) and the guid-keep guard (_apply_item preserves a prior guid when the listing
  carries none, still updates on a real one).

Full backend suite 276 passed / 2 skipped.

backend/app/plex/sync.py

@@ -463,11 +463,13 @@ def _sync_overlay(
     for group in by_guid.values():
         for row in group:
             row.title_hu = row.summary_hu = row.genres_hu = None
+    seen: set[str] = set()  # first HU entry for a guid wins; a dup HU item can't overwrite or double-count
     for meta in _paginate(plex, lib.plex_key, item_type):
         g = _canonical_guid(meta)
         targets = by_guid.get(g) if g else None
-        if not targets:
+        if not targets or g in seen:
             continue
+        seen.add(g)
         title_hu = (meta.get("title") or "").strip() or None
         summary_hu = meta.get("summary")
         genres_hu = _tags(meta, "Genre")

backend/tests/test_plex_overlay.py

@@ -10,7 +10,7 @@ from sqlalchemy import func, select
 import app.plex.sync as syncmod
 from app import sysconfig
 from app.models import PlexItem, PlexLibrary
-from app.plex.sync import _canonical_guid, _sync_overlay
+from app.plex.sync import _apply_item, _canonical_guid, _sync_overlay
 from app.routes.plex.browse import _content_libs
 
 
@@ -170,6 +170,42 @@ def test_overlay_reconcile_clears_stale_hu_when_twin_vanishes(db):
     assert row.title_hu is None and row.summary_hu is None and row.genres_hu is None
 
 
+def test_apply_item_keeps_prior_guid_when_listing_lacks_guid():
+    row = PlexItem(rating_key="m1", kind="movie", guid="tmdb:1")
+    _apply_item(row, lib_id=7, kind="movie", meta={"title": "Heat"})  # no Guid array
+    assert row.guid == "tmdb:1"  # the prior guid survives a guid-less listing
+    _apply_item(row, lib_id=7, kind="movie", meta={"title": "Heat", "Guid": [{"id": "tmdb://2"}]})
+    assert row.guid == "tmdb:2"  # a real guid in the listing still updates it
+
+
+def test_overlay_applies_to_all_editions_sharing_a_guid(db):
+    en_lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
+    hu_lib = PlexLibrary(plex_key="9", title="Filmek", kind="movie", metadata_lang="hu")
+    db.add_all([en_lib, hu_lib])
+    db.flush()
+    hu_lib.overlay_of = en_lib.id
+    ed1 = PlexItem(rating_key="a", library_id=en_lib.id, kind="movie", title="Heat", guid="tmdb:1")
+    ed2 = PlexItem(rating_key="b", library_id=en_lib.id, kind="movie", title="Heat (Director's Cut)", guid="tmdb:1")
+    db.add_all([ed1, ed2])
+    db.commit()
+
+    # Two HU entries share the guid — first wins, no double count.
+    stub = _StubPlex(
+        [
+            _hu_movie("h1", "Öldöklés", "első", ["Akció"], "tmdb://1"),
+            _hu_movie("h2", "Öldöklés (dup)", "második", ["Krimi"], "tmdb://1"),
+        ]
+    )
+    stats = {"overlaid": 0}
+    _sync_overlay(db, stub, hu_lib, en_lib.id, stats)
+    db.commit()
+    db.refresh(ed1)
+    db.refresh(ed2)
+    assert ed1.title_hu == "Öldöklés" and ed2.title_hu == "Öldöklés"  # both editions overlaid
+    assert ed1.genres_hu == ["Akció"] and ed2.genres_hu == ["Akció"]  # first HU entry won
+    assert stats["overlaid"] == 1  # counted once for the guid, not per edition or per dup HU item
+
+
 def test_content_libs_excludes_overlay_libraries(db):
     en_lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
     en_shows = PlexLibrary(plex_key="1", title="TV Shows", kind="show")