Hello, thanks for the reply.
That's true, I haven't thought of that through as much. But I don't think it will be much of a problem, if the user wanted for example to add a song to two different playlists, they would have to show all the songs (even ones in playlists) to revise and add it to more than a playlist.
My idea was mostly referring to it being an option just like the sorting and filtering options.
Thanks for the swift reply, cheers.