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I sometimes forget that I have a song already in a playlist and when I add a song already in my playlist it will duplicate it and not let me know its already in a specific playlist I am choosing to add it.
I can never remember if a pandora generated song is already on one of my playlists, so I add it just to be sure. I was thinking that listing or highlighting songs as already being on a playlist, either on the now playing screen or highlighting the song or showing what playlist the song is already part of in the add to playlist feature might be helpful.