When trying to submit an editor session that edits a relationship to be the same as another existing one (for a relatively common example, when you have a relationship with and without dates and accidentally copy the dates with a script to the one without), an error message "This relationship already exists" is shown at the bottom of the relationship editor. This is all the info the user gets though - if they have changed hundreds of relationships, they will have to try and review everything to figure out which one might be the duplicate. We should provide some context (at least which entities are involved in the relationship) to make this less frustrating.