If you create a playlist with duration (ms) decending/ascending this should show you the tracks that have the same play time = duplicates? I have tested it and this does pick them up. Of course, there are tracks that so happen to have the same duration and to filter those you could click on Artist column inside the playlist. You should see all the same tracks that are by the same artists, same duration and the multiples. I wish there were more columns to add to help filter playlists but sadly not. I will have to go year by year as we also have the 1000 song limit to deal with too...
Google music manager has uploaded a ton of my tracks because of small tag changes. The title, album, artist and genre tags are not changed, but looking at some of these duplicates in my google music library, the changes I have made to composer, comment and track number tags have caused these duplicates. I am about ready to nuke the lot and reupload the 40000 odd tracks in my library if I can't get this playlist by duration (ms) to meet my needs.
Cheers