I need to read up on the various tagging standards and best practices. I only have a limited understanding of the details. The Vorbis doc was informative. I'll have to read up on ID3 info.
The Vorbis doc says it is permissible to have multiple entries of the same field name:
Field names are not required to be unique (occur once) within a comment header. As an example, assume a track was recorded by three well know artists; the following is permissible, and encouraged:
ARTIST=Dizzy Gillespie
ARTIST=Sonny Rollins
ARTIST=Sonny Stitt
So the files I have with two PUBLISHER tags aren't wrong. It causes unexpected behavior when it is assumed to be a single entry field, but is not wrong.
In my little experimentation is seems that MC is reluctant to read in the PUBLISHER tag even if the PUBLISHER field in the database is empty. Even after doing a "update library from tags". I find that odd, but there is likely a reason for that behavior.
I manually entered label and catalog numbers for some albums in the PUBLISHER tag using Mp3tag. I also entered the same info in the COMMENT tag so even if the PUBLISHER tag gets whacked or MC doesn't want to read it, the info is still there in the COMMENT tag and should be safe.