Why do you care if someone wants to add the same song more than once to a playlist? You create your own playlist, don't you? Unless I am misunderstanding your post (possible), you don't want anyone to add the same song more than once to a single playlist because it somehow impacts you. And yet, when YOU make up your playlists, you have complete control over how many times you add a song. You mistakenly hit the + twice, you can just erase it from the list. You mistakenly add the same song later on, you can just erase it. So what is the problem?