To reproduce:

• Add a new artist without a disambiguation comment
• Add a second artist with the same name, which will require a disambiguation comment
• Edit the first artist and add the same disambiguation comment
• Submit the edit

Expected result: The edit page shows an error, because there is already an artist with that name and disambiguation comment
Actual result: An edit is created which fails with the status "Internal Error", as happened in https://musicbrainz.org/edit/29291077