fix(scanner): prevent duplicate tracks when multiple missing files match same target (#5183)

In processMissingTracks, matched tracks were not removed from the candidate
pool after being consumed by moveMatched. This allowed the same target track
to be paired with multiple missing tracks, creating duplicate non-missing
records with the same path. Track consumed matches in a usedMatched map so
each target is used at most once.

Fixes #5169

scanner/phase_2_missing_tracks.go

@@ -114,6 +114,10 @@ func (p *phaseMissingTracks) stages() []ppl.Stage[*missingTracks] {
 
 func (p *phaseMissingTracks) processMissingTracks(in *missingTracks) (*missingTracks, error) {
 	hasMatches := false
+	// Track which matched entries have already been consumed, so each matched track
+	// is only used once. Without this, the same matched track could be paired with
+	// multiple missing tracks, creating duplicate records with the same path.
+	usedMatched := make(map[string]bool, len(in.matched))
 
 	for _, ms := range in.missing {
 		var exactMatch model.MediaFile
@@ -121,6 +125,9 @@ func (p *phaseMissingTracks) processMissingTracks(in *missingTracks) (*missingTr
 
 		// Identify exact and equivalent matches
 		for _, mt := range in.matched {
+			if usedMatched[mt.ID] {
+				continue
+			}
 			if ms.Equals(mt) {
 				exactMatch = mt
 				break // Prioritize exact match
@@ -138,13 +145,14 @@ func (p *phaseMissingTracks) processMissingTracks(in *missingTracks) (*missingTr
 				log.Error(p.ctx, "Scanner: Error moving matched track", "missing", ms.Path, "movedTo", exactMatch.Path, "lib", in.lib.Name, err)
 				return nil, err
 			}
+			usedMatched[exactMatch.ID] = true
 			p.totalMatched.Add(1)
 			hasMatches = true
 			continue
 		}
 
 		// If there is only one missing and one matched track, consider them equivalent (same PID)
-		if len(in.missing) == 1 && len(in.matched) == 1 {
+		if len(in.missing) == 1 && len(in.matched) == 1 && !usedMatched[in.matched[0].ID] {
 			singleMatch := in.matched[0]
 			log.Debug(p.ctx, "Scanner: Found track with same persistent ID in a new place", "missing", ms.Path, "movedTo", singleMatch.Path, "lib", in.lib.Name)
 			err := p.moveMatched(singleMatch, ms)
@@ -152,6 +160,7 @@ func (p *phaseMissingTracks) processMissingTracks(in *missingTracks) (*missingTr
 				log.Error(p.ctx, "Scanner: Error updating matched track", "missing", ms.Path, "movedTo", singleMatch.Path, "lib", in.lib.Name, err)
 				return nil, err
 			}
+			usedMatched[singleMatch.ID] = true
 			p.totalMatched.Add(1)
 			hasMatches = true
 			continue
@@ -165,6 +174,7 @@ func (p *phaseMissingTracks) processMissingTracks(in *missingTracks) (*missingTr
 				log.Error(p.ctx, "Scanner: Error updating matched track", "missing", ms.Path, "movedTo", equivalentMatch.Path, "lib", in.lib.Name, err)
 				return nil, err
 			}
+			usedMatched[equivalentMatch.ID] = true
 			p.totalMatched.Add(1)
 			hasMatches = true
 		}

scanner/phase_2_missing_tracks_test.go

@@ -241,6 +241,39 @@ var _ = Describe("phaseMissingTracks", func() {
 			Expect(movedTrack.Size).To(Equal(missingTrack.Size))
 		})
 
+		It("should not match the same target to multiple missing tracks (prevents duplicate paths)", func() {
+			// Simulate a scenario where two missing tracks from different locations have the same
+			// base filename and match the same newly imported track via IsEquivalent.
+			// Without deduplication, both missing tracks would be "moved" to the same target,
+			// creating two non-missing records with the same path.
+			missingTrack1 := model.MediaFile{ID: "1", PID: "A", Path: "old_dir1/song.mp3", Title: "title1", Size: 100}
+			missingTrack2 := model.MediaFile{ID: "2", PID: "A", Path: "old_dir2/song.mp3", Title: "title1", Size: 100}
+			matchedTrack := model.MediaFile{ID: "3", PID: "A", Path: "new_dir/song.mp3", Title: "title1", Size: 200}
+
+			_ = ds.MediaFile(ctx).Put(&missingTrack1)
+			_ = ds.MediaFile(ctx).Put(&missingTrack2)
+			_ = ds.MediaFile(ctx).Put(&matchedTrack)
+
+			in := &missingTracks{
+				missing: []model.MediaFile{missingTrack1, missingTrack2},
+				matched: []model.MediaFile{matchedTrack},
+			}
+
+			_, err := phase.processMissingTracks(in)
+			Expect(err).ToNot(HaveOccurred())
+			// Only one of the missing tracks should be matched
+			Expect(phase.totalMatched.Load()).To(Equal(uint32(1)))
+			Expect(state.changesDetected.Load()).To(BeTrue())
+
+			// The matched track should have been consumed by the first missing track
+			movedTrack, _ := ds.MediaFile(ctx).Get("1")
+			Expect(movedTrack.Path).To(Equal(matchedTrack.Path))
+
+			// The second missing track should remain unchanged
+			unmatchedTrack, _ := ds.MediaFile(ctx).Get("2")
+			Expect(unmatchedTrack.Path).To(Equal(missingTrack2.Path))
+		})
+
 		It("should return an error when there's an error moving the matched track", func() {
 			missingTrack := model.MediaFile{ID: "1", PID: "A", Path: "path1.mp3", Tags: model.Tags{"title": []string{"title1"}}}
 			matchedTrack := model.MediaFile{ID: "2", PID: "A", Path: "path1.mp3", Tags: model.Tags{"title": []string{"title1"}}}