I tried to edit an instruments: piano rel to be a vocals: spoken one by a different artist (some track in a piano album turned out to be a spoken introduction). The relationship editor ended up with two relationships instead - it added a new "spoken vocals" one (https://musicbrainz.org/edit/123450844) which was what I was trying to edit the existing relationship to be, plus it edited the existing one to just "vocals" (https://musicbrainz.org/edit/123450845).

The two relationships were shown at the same time before submission, but there was no good way to fix the problem because removing the "new" spoken rel and trying to edit the other one would, IIRC, lead to the same effect.