Thanks for the replies.
I had already looked for a duplicate file structure, and could find no such thing; there was no "extra" music folder, only one music folder plus its 100s of subfolders. No sign of duplicate artists, albums etc folders. It wasn't until I drilled down to individual albums that the duplicates appeared. If there was some minor difference in the file strings of 2 identical songs, I hadn't actually checked as I was not prepared to individually go into 1000 different albums to delete them at that level. Well, it was actually only a seemingly random 1/3 of albums.
Fortunately I do have a backup of my playlist m3u8 files from a few months ago. I'd hardly changed them recently as I find it really difficult to add files to playlists on a tiny touch phone. No easy way to select multiple songs across years, artists, etc. That leads to another question - is it possible for me to run Poweramp on a desktop PC, to create and edit playlists there? So much easier for me to work with my 10,000 song collection on a full screen computer with a mouse.
Greg