I copied my entire itunes music folder over to the internal 1TB drive (which was completely empty before), but now I have loads of duplicate music tracks within each album! Sometimes 2 of each track, sometimes 5 or 6! And it’s not consistent - for example the album I’m browsing now shows 2 copies of every track, down to the last 10 tracks, which only show once each. Another album has a mixture of 1, 2, 3 or 5 for various tracks.
I checked the folder for each album and there’s definitely only 1 copy of each mp3 or m4a file.