Add a test to reproduce the issue

test/autotag/test_match.py

@@ -1,6 +1,9 @@
+from typing import ClassVar
+
 import pytest
 
-from beets.autotag import TrackInfo, match
+from beets import metadata_plugins
+from beets.autotag import AlbumInfo, TrackInfo, match
 from beets.library import Item
 
 
@@ -92,3 +95,106 @@ class TestAssignment:
         expected = list(zip(items, trackinfo)), [], []
 
         assert match.assign_items(items, trackinfo) == expected
+
+
+class TestTagMultipleDataSources:
+    @pytest.fixture
+    def shared_track_id(self):
+        return "track-12345"
+
+    @pytest.fixture
+    def shared_album_id(self):
+        return "album-12345"
+
+    @pytest.fixture(autouse=True)
+    def _setup_plugins(self, monkeypatch, shared_album_id, shared_track_id):
+        class StubPlugin(metadata_plugins.MetadataSourcePlugin):
+            @property
+            def track(self):
+                return TrackInfo(
+                    artist="Artist",
+                    title="Title",
+                    track_id=shared_track_id,
+                    data_source=self.data_source,
+                )
+
+            @property
+            def album(self):
+                return AlbumInfo(
+                    [self.track],
+                    artist="Albumartist",
+                    album="Album",
+                    album_id=shared_album_id,
+                    data_source=self.data_source,
+                )
+
+            def album_for_id(self, *_):
+                return self.album
+
+            def track_for_id(self, *_):
+                return self.track
+
+            def candidates(self, *_, **__):
+                yield self.album
+
+            def item_candidates(self, *_, **__):
+                yield self.track
+
+        class DeezerPlugin(StubPlugin):
+            pass
+
+        class DiscogsPlugin(StubPlugin):
+            pass
+
+        monkeypatch.setattr(
+            metadata_plugins,
+            "find_metadata_source_plugins",
+            lambda: [DeezerPlugin(), DiscogsPlugin()],
+        )
+
+    def check_proposal(self, proposal):
+        sources = [
+            candidate.info.data_source for candidate in proposal.candidates
+        ]
+        assert len(sources) == 2
+        assert set(sources) == {"Discogs", "Deezer"}
+
+    @pytest.mark.xfail(
+        reason="Album ID collisions drop extra sources (#6177)",
+        raises=AssertionError,
+        strict=True,
+    )
+    def test_search_album_ids(self, shared_album_id):
+        _, _, proposal = match.tag_album([Item()], search_ids=[shared_album_id])
+
+        self.check_proposal(proposal)
+
+    @pytest.mark.xfail(
+        reason="Album ID collisions drop extra sources (#6177)",
+        raises=AssertionError,
+        strict=True,
+    )
+    def test_search_album_current_id(self, shared_album_id):
+        _, _, proposal = match.tag_album([Item(mb_albumid=shared_album_id)])
+
+        self.check_proposal(proposal)
+
+    @pytest.mark.xfail(
+        reason="Track ID collisions drop extra sources (#6177)",
+        raises=AssertionError,
+        strict=True,
+    )
+    def test_search_track_ids(self, shared_track_id):
+        proposal = match.tag_item(Item(), search_ids=[shared_track_id])
+
+        self.check_proposal(proposal)
+
+    @pytest.mark.xfail(
+        reason="Track ID collisions drop extra sources (#6177)",
+        raises=AssertionError,
+        strict=True,
+    )
+    def test_search_track_current_id(self, shared_track_id):
+        proposal = match.tag_item(Item(mb_trackid=shared_track_id))
+
+        self.check_proposal(proposal)