fix(playlists): preserve repeated tracks during imports

lib/models/library_collections.dart

@@ -23,6 +23,22 @@ String trackCollectionKey(Track track) => libraryTrackCollectionKey(
   isrc: track.isrc,
 );
 
+/// Stable entry keys preserve repeated positions when importing a playlist.
+/// The first occurrence keeps the existing membership key, so re-importing
+/// fills missing occurrences without appending the entire playlist again.
+Iterable<String> playlistImportKeys(Iterable<Track> tracks) sync* {
+  final occurrences = <String, int>{};
+  for (final track in tracks) {
+    final key = trackCollectionKey(track);
+    final count = occurrences.update(
+      key,
+      (value) => value + 1,
+      ifAbsent: () => 1,
+    );
+    yield count == 1 ? key : 'occurrence:$count:$key';
+  }
+}
+
 String collectionArtistId(String value) {
   final colonIndex = value.indexOf(':');
   if (colonIndex <= 0 || colonIndex == value.length - 1) {
@@ -255,10 +271,14 @@ class PlaylistPickerSummaryRequest {
 
   PlaylistPickerSummaryRequest._(this.trackKeys);
 
-  factory PlaylistPickerSummaryRequest.fromTracks(Iterable<Track> tracks) {
+  factory PlaylistPickerSummaryRequest.fromTracks(
+    Iterable<Track> tracks, {
+    bool preserveOccurrences = false,
+  }) {
     final keys =
-        tracks
-            .map(trackCollectionKey)
+        (preserveOccurrences
+                ? playlistImportKeys(tracks)
+                : tracks.map(trackCollectionKey))
             .where((key) => key.trim().isNotEmpty)
             .toSet()
             .toList(growable: false)

lib/providers/library_collections_provider.dart

@@ -519,8 +519,9 @@ class LibraryCollectionsNotifier extends Notifier<LibraryCollectionsState> {
 
   Future<PlaylistAddBatchResult> addTracksToPlaylist(
     String playlistId,
-    Iterable<Track> tracks,
-  ) => _mutate(() async {
+    Iterable<Track> tracks, {
+    bool preserveOccurrences = false,
+  }) => _mutate(() async {
     await ensurePlaylistLoaded(playlistId);
     if (!ref.mounted) {
       return const PlaylistAddBatchResult(
@@ -541,6 +542,7 @@ class LibraryCollectionsNotifier extends Notifier<LibraryCollectionsState> {
       tracks: tracks,
       existingKeys: playlist.trackKeys,
       addedAt: now,
+      preserveOccurrences: preserveOccurrences,
     );
     try {
       if (!ref.mounted || prepared.entries.isEmpty) {

lib/screens/playlist_screen.dart

@@ -558,6 +558,7 @@ class _PlaylistScreenState extends ConsumerState<PlaylistScreen>
               ref,
               _tracks,
               playlistNamePrefill: widget.playlistName,
+              preserveOccurrences: true,
             ),
     );
   }

lib/services/collection_track_batch.dart

@@ -44,6 +44,7 @@ Future<PreparedCollectionTrackBatch> prepareCollectionTrackBatch({
   required Iterable<Track> tracks,
   required Set<String> existingKeys,
   required DateTime addedAt,
+  bool preserveOccurrences = false,
   bool? useNative,
   Directory? temporaryDirectory,
 }) async {
@@ -58,10 +59,22 @@ Future<PreparedCollectionTrackBatch> prepareCollectionTrackBatch({
   }
   try {
     if (snapshot.length < _workerThreshold) {
-      return _prepare(snapshot, existingKeys, addedAt, inputDirectory);
+      return _prepare(
+        snapshot,
+        existingKeys,
+        addedAt,
+        inputDirectory,
+        preserveOccurrences,
+      );
     }
     return await Isolate.run(
-      () => _prepare(snapshot, existingKeys, addedAt, inputDirectory),
+      () => _prepare(
+        snapshot,
+        existingKeys,
+        addedAt,
+        inputDirectory,
+        preserveOccurrences,
+      ),
     );
   } catch (_) {
     if (inputDirectory != null) await _removeDirectory(inputDirectory);
@@ -74,12 +87,17 @@ PreparedCollectionTrackBatch _prepare(
   Set<String> existingKeys,
   DateTime addedAt,
   String? inputDirectory,
+  bool preserveOccurrences,
 ) {
   final keys = <String>{};
   final entries = <CollectionTrackEntry>[];
   var duplicates = 0;
+  final importKeys = preserveOccurrences
+      ? playlistImportKeys(tracks).iterator
+      : null;
   for (final track in tracks) {
-    final key = trackCollectionKey(track);
+    importKeys?.moveNext();
+    final key = importKeys?.current ?? trackCollectionKey(track);
     if (existingKeys.contains(key) || !keys.add(key)) {
       duplicates++;
     } else {

lib/widgets/playlist_picker_sheet.dart

@@ -29,6 +29,7 @@ Future<void> showAddTracksToPlaylistSheet(
   WidgetRef ref,
   List<Track> tracks, {
   String? playlistNamePrefill,
+  bool preserveOccurrences = false,
 }) async {
   if (tracks.isEmpty) return;
 
@@ -44,6 +45,7 @@ Future<void> showAddTracksToPlaylistSheet(
       final content = _PlaylistPickerSheetContent(
         tracks: tracks,
         playlistNamePrefill: playlistNamePrefill,
+        preserveOccurrences: preserveOccurrences,
       );
       return sheetContext.isMornye
           ? MornyeGlassPanel(tintOpacity: 0.78, child: content)
@@ -55,10 +57,12 @@ Future<void> showAddTracksToPlaylistSheet(
 class _PlaylistPickerSheetContent extends ConsumerStatefulWidget {
   final List<Track> tracks;
   final String? playlistNamePrefill;
+  final bool preserveOccurrences;
 
   const _PlaylistPickerSheetContent({
     required this.tracks,
     this.playlistNamePrefill,
+    required this.preserveOccurrences,
   });
 
   @override
@@ -76,7 +80,10 @@ class _PlaylistPickerSheetContentState
   @override
   void initState() {
     super.initState();
-    _summaryRequest = PlaylistPickerSummaryRequest.fromTracks(widget.tracks);
+    _summaryRequest = PlaylistPickerSummaryRequest.fromTracks(
+      widget.tracks,
+      preserveOccurrences: widget.preserveOccurrences,
+    );
   }
 
   Future<void> _handleDone(List<PlaylistPickerSummary> playlists) async {
@@ -96,7 +103,11 @@ class _PlaylistPickerSheetContentState
 
     try {
       for (final playlistId in idsToAdd) {
-        await notifier.addTracksToPlaylist(playlistId, widget.tracks);
+        await notifier.addTracksToPlaylist(
+          playlistId,
+          widget.tracks,
+          preserveOccurrences: widget.preserveOccurrences,
+        );
         _committedPlaylistIds.add(playlistId);
         final playlistName = playlistNamesById[playlistId];
         if (playlistName != null && playlistName.isNotEmpty) {
@@ -217,7 +228,11 @@ class _PlaylistPickerSheetContentState
                 return;
               }
               final playlistId = await notifier.createPlaylist(name.trim());
-              await notifier.addTracksToPlaylist(playlistId, widget.tracks);
+              await notifier.addTracksToPlaylist(
+                playlistId,
+                widget.tracks,
+                preserveOccurrences: widget.preserveOccurrences,
+              );
               if (!mounted) return;
               setState(() {
                 _committedPlaylistIds.add(playlistId);