@dflat I have the same issue as the OP since I get sent numerous playlists from many different people or for different gigs, I often end up with duplicates.
Also it would be useful to know which playlist a song is in as it would help me to work out who sent me each song.
Also when creating a playlist myself it would be useful to see if a particular tune is already in my playlist so I don’t try adding it again.

I don’t see that your suggested solution would help for these things (although thanks as ever for suggesting!) - what would be really useful would be if when you click on Add to Playlist, it brings up the list of existing playlists, if there was a small tick next to the playlists that song is already in.

Would that be possible to add?
Thanks!