Entities in a relationship cannot be the same. This is a general assumption used in the code. This assumption is checked twice: first in relationship editor dialog (coded in 2015-01), then in relationship edit creation (coded in 2014). However, there is not check in the database. This somehow allowed to enter a loop relationship with https://musicbrainz.org/edit/32309519 (in 2015-03), thus causing troubles reported in MBS-9663.