It would be convenient when adding a song to a playlist if a warning popped up notifying the user when they are about to add a duplicate entry. Otherwise, it would be convenient (but less so) to have the option to right-click a playlist to "Remove duplicate entries".

I often play songs on shuffle from my large collection and stumble upon something I wish to add to an existing, large playlist. At the moment I just add it and hope it's not a duplicate, but in many cases I discover I've added duplicate entries. If the list is small I sometimes navigate to the playlist first to see that the song is not already there and then I go back and add it, but this is fairly inconvenient.