Handle dups of existing genres in lastgenre plugin

When handling existing comma-separated genres in the _get_genre method
of the plugin, make sure duplicate genres are removed.

beetsplug/lastgenre/__init__.py

@@ -325,6 +325,7 @@ def _get_genre(self, obj):
                 if allowed:
                     keep_allowed.append(g)
             if keep_allowed:
+                keep_allowed = set(keep_allowed)
                 return ", ".join(keep_allowed), "keep"
 
         # Track genre (for Items only).