According to the discussion in https://community.metabrainz.org/t/wikidata-item-for-single-rg-single-song-or-both/557322 there seems to be an edict that a Wikidata link should only exist on one type of MusicBrainz entity, i.e., a Wikidata link shouldn't be shared between a MusicBrainz Work and a MusicBrainz Release Group.

If this is in fact the case, then the editing interface should warn the user if the Wikidata link already exists in MusicBrainz.