eg. after the issue with https://bookbrainz.org/work/65e71f2e-7245-42df-b93e-89463a28f75c being merged, also it still has relationship pointing to itself

it should be impossible to create this, either directly or via merge

I think perhaps it should block a merge - we can throw up a warning and add "if these two [entity] are merged, then it will link to itself, are you sure you want to merge these?" or something like this