http://chatlogs.metabrainz.org/brainzbot/metabrainz/2016-03-26/?msg=3550090&page=4

After some investigation, we found that it's sending an empty mbid parameter to /ws/js/check_duplicates, so https://github.com/metabrainz/musicbrainz-server/blob/f630a14/lib/MusicBrainz/Server/Controller/WS/js/CheckDuplicates.pm#L43-L43 isn't working.

Not sure why it's doing that. It should take the mbid from "MB.sourceEntityGID" on the page, which is set by the relationship editor code, and that code should always run before the duplicate-checking code. We can try to have it get the mbid from the window.location href instead.