After testing how it might have happened that the user in question created a duplicate artist, I found the following:
1. when doing a freedb import, the release editor doesn't select the artist until the 'add missing entities' step.
2. by default, no radio button is selected.
3. If no radio button is selected it apparently defaults to 'create new artist', forcing the user to enter a sortname and a disambiguation, instead of forcing them to choose a radio button
Also, IMO the sortname/disambiguation should only be shown if the user has already chosen to create a new artist.