Hey Steve,
Been a while since I posted here.
I realised, when you add a song to a PL, by default a pop will let you know if it's a duplicate in the list, you can chose to add it or not, with an option to tick DO THAT FROM NOW ON.
However, that ticking doesn't do anything - if I say 'no don't add the duplicate' and try that with another song, the pop will not appear but the song will be added nonetheless.
Also, the duplicate finder from the MENU, when in the Playlist view, needs to delete the file ONLY from the list and not from the disk itself. If we want to do that we would do the duplicate finding from the Music Library itself, wouldn't we?
Hope things are going well for you. Thanks again.