[Warner306]

The pair of artist or album artist (name string) and mb artist id or mb album artist id is treated as a unique identifier, unless you use the setting to "auto-correct" the name string, in which case the mb artist id or mb album artist id will be used to obtain the name string from mb database (I don't use this setting and I'm not sure if this can cause dupes or not -- would need to do some testing). I use mp3tag to set or review mb ids.

My rule of thumb is that if you have any file with mb artist or album artist id for an artist, you must ensure that all files with that artist also have the same mb artist or album artist id. There was a bug where the tag scanner was not reading the id properly when there were multiple artists (and multiple mb artist ids) in a file tag and that was causing problems, but that is fixed. Note that the order that artists appear in a file tag must match the order of the mb artist ids -- there isn't any other pairing mechanism so first artist is paired to first mb artist id, second with second, etc.

scott s.
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So to summarize, the Album Artist and Album Artist ID must match and be in the same order for every file for that artist. Similarly, the Artist and Artist ID must match and be in the same order for every song. I use mp3tag, but any similar tagging program such as Picard can be used to correct individual tags. The tags in question come from MusicBrainz.