lib: Don't allow adding a track to a tag multiple times

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

lib/tag.py

@@ -67,6 +67,8 @@ class Tag:
 
     def add_track(self, track):
         with self.lock:
+            if track in self.tracks:
+                return
             pos = len(self.tracks)
             self.tracks.append(track)
             self.runtime += track.length

lib/test_tag.py

@@ -108,6 +108,11 @@ class TestTag(unittest.TestCase):
         self.assertEqual(self.changed, (t, fake.Track(2), 1))
         self.assertEqual(t.runtime, 3)
 
+        t.add_track(fake.Track(1))
+        self.assertEqual(t.tracks, [ fake.Track(1), fake.Track(2) ])
+        self.assertEqual(self.changed, (t, fake.Track(2), 1))
+        self.assertEqual(t.runtime, 3)
+
     def test_tag_remove_track(self):
         t = tag.Tag("test")
         t.add_track(fake.Track(1))