When adding a song to a playlist you dont have any idea whether or not if that song is already added to that playlist which is really cumbersome as you have to go around and manually clean up all duplicates every now and then. Some indication if that song already is added to a playlist would be really helpful.
A checkmark or some sort of indication that the song is added to a playlist would solve the problem.
Another idea is that it could also show how many instances of a song exists in the playlist if there are more than one, by showing a number instead of a basic indication when there are more than one instance.
It would save a lot of time.
Spotify solves it by just showing a popup informing you that song is added to the playlist. However, I think my proposition is more convenient. Also you have previously noted that spotifys way of doing it is not possible in symfonium.