Cover Art Filler: skip files that already have art (keep apply purely additive)

Verification found a non-additive edge: embed_album_art_metadata uses FLAC
add_picture(), which APPENDS — so applying to an album where some tracks already
had art would have added a duplicate embedded picture. The apply now checks each
file and skips any that already carry art (shared _audio_has_art helper), so it
only ever ADDS art to files missing it. Test covers the skip (no re-embed).

core/metadata/art_apply.py

@@ -53,29 +53,33 @@ def file_has_embedded_art(file_path: str) -> bool:
     if not symbols:
         return False
     try:
-        audio = symbols.File(file_path)
-        if audio is None:
-            return False
-        # FLAC / Ogg expose picture blocks directly.
-        if getattr(audio, "pictures", None):
-            return True
-        tags = getattr(audio, "tags", None)
-        if isinstance(audio, symbols.MP4):
-            return bool(audio.get("covr"))
-        if tags is None:
-            return False
-        with contextlib.suppress(Exception):
-            if isinstance(tags, symbols.ID3):
-                return bool(tags.getall("APIC"))
-        with contextlib.suppress(Exception):
-            if "metadata_block_picture" in tags:
-                return True
-        return False
+        return _audio_has_art(symbols.File(file_path), symbols)
     except Exception as exc:
         logger.debug("art presence check failed for %s: %s", file_path, exc)
         return False
 
 
+def _audio_has_art(audio, symbols) -> bool:
+    """True if an already-open mutagen object carries embedded cover art."""
+    if audio is None:
+        return False
+    # FLAC / Ogg expose picture blocks directly.
+    if getattr(audio, "pictures", None):
+        return True
+    if isinstance(audio, symbols.MP4):
+        return bool(audio.get("covr"))
+    tags = getattr(audio, "tags", None)
+    if tags is None:
+        return False
+    with contextlib.suppress(Exception):
+        if isinstance(tags, symbols.ID3):
+            return bool(tags.getall("APIC"))
+    with contextlib.suppress(Exception):
+        if "metadata_block_picture" in tags:
+            return True
+    return False
+
+
 def album_has_art_on_disk(rep_file_path: str) -> bool:
     """Does this album have art on disk?
 
@@ -123,6 +127,12 @@ def apply_art_to_album_files(
             if audio is None:
                 result["skipped"] += 1
                 continue
+            # Purely additive: never touch a file that already has art. Embedding
+            # again would APPEND a duplicate picture on FLAC (add_picture doesn't
+            # replace), so leave already-arted files alone.
+            if _audio_has_art(audio, symbols):
+                result["skipped"] += 1
+                continue
             # ID3 needs a tag container before APIC can be added.
             if getattr(audio, "tags", None) is None and hasattr(audio, "add_tags"):
                 with contextlib.suppress(Exception):

tests/test_art_apply.py

@@ -119,6 +119,22 @@ def test_apply_embeds_into_each_file_and_writes_cover(tmp_path, monkeypatch):
     assert res['cover_written'] is True
 
 
+def test_apply_skips_files_that_already_have_art(tmp_path, monkeypatch):
+    """Purely additive: a file that already has art is left untouched (no
+    duplicate FLAC picture, no re-embed)."""
+    f1 = tmp_path / '01.flac'; f1.write_bytes(b'')
+    audio = SimpleNamespace(pictures=['existing'], tags=None, save=lambda: None)
+    monkeypatch.setattr(aa, 'get_mutagen_symbols', lambda: _fake_symbols(audio))
+    embed_calls = []
+    monkeypatch.setattr(aa, 'embed_album_art_metadata', lambda a, m: embed_calls.append(m) or True)
+    monkeypatch.setattr(aa, 'download_cover_art', lambda *a, **k: None)
+
+    res = aa.apply_art_to_album_files([str(f1)], {'album': 'B'}, {'album_name': 'B'}, folder=str(tmp_path))
+    assert res['embedded'] == 0
+    assert res['skipped'] == 1
+    assert embed_calls == []      # never re-embedded → no duplicate picture
+
+
 def test_apply_counts_failures_without_raising(tmp_path, monkeypatch):
     f1 = tmp_path / '01.flac'; f1.write_bytes(b'')
     audio = SimpleNamespace(tags=object(), save=lambda: (_ for _ in ()).throw(OSError('read-only')))