This might be just a residual effect of an (resolved) amarok bug (see KDE bug #87986). I had duplicate album entries for AFAIK every mp3 file on my NTFS data partition. Less annoying, but equally weird: playlists in the playlist browser were listed in triplicate. Deleting playlist files and rescanning the collection (even getting Amarok to scan a completely different folder) had no effect.
What worked was to quit Amarok, remove the following files from ~/.kde/share/apps/amarok - collection.db, collection_scan.files, collection_scan.log (can't hurt ;-)) and playlistbrowser_save.xml - restart and rescan collection. After that, rescanning, changing folders, adding new files and changing tags worked OK.
Another bit of weirdness: apparently I can't create playlists on the NTFS partition (Amarok complains "You can only select local files"), but overwriting existing ones works. Edit: That's actually a (resolved) NTFS-3g problem, get the last stable release (no, not 1.913!!!). Thanks to Sven from roKymotion for pointing that out.