After merging, either of the original mbids for an entity will resolve to the same thing. Right now this can only be detected by a webservice client by noticing that the mbid returned by the webservice is the same for both requests, an assumption that is not necessarily valid (but is valid with the current server implementation).

A better solution would be to include all of the mbids when responding to a request.

This also solves an issue in my player, where it caches data locally; if someone adds a new recording and a previous recording that it had loaded was merged in to the same recording in the meantime, it could detect the merge by noticing that it already had a recording for one of the returned mbids out of the list.

In order to be fully useful, this would also have to include all mbids for linked entities as well; e.g. if you do a release query with inc=recordings+artists, it should include all mbids for each recording and artist.