Inspired by MBS-12432

It's pretty common for the copyright line and often the label field, if there is such, to use the artist's name as placeholder for self-releases. 

The guidelines were clarified that by default the special purpose label [no label] should be used in those cases:
https://tickets.metabrainz.org/browse/STYLE-1447

It seems sensible to warn the user in the UI rather than expect each of the many userscripts and tools like a-tisket to implement such a check.

A possible override for the warning could be the presence of "personal label" or personal publisher" relationship to the artist matching the name.