I'm completely on board with @psychoadept. The rules I set for "manage duplicates" should apply to duplicate selection across the board. At the very least, the duplicate warning should give more context on the supposedly duplicate files, and allow either "select all" or selection on a case-by-case basis. There are, sure, some circumstances when I may not realize I'm adding a song to a playlist that was already on there, but if I'm adding a greatest hits album to a playlist (I don't really have very many of them, but I can see it as a possible scenario for other people), I'm not going to want to add the album piecemeal just because one song on it is also on another album I've added to the playlist.