Exactly like MBS-4994.
This applies to:
artist name
artist sortname
artist comment
artist alias name
artist alias sortname
release group name
release group comment
release group artist credit artist name
release name
release comment
release artist credit artist name
medium name
recording name
recording comment
recording artist credit artist name
track name
track number
work name
work comment
work alias name
work alias sortname
label name
label sortname
label comment
label alias name
label alias sortname
With the exception of track name, all of those appear to be free from leading, trailing and double spaces.
In review with http://codereview.musicbrainz.org/r/2130/diff/#index_header