grl-acoustid: Do not return the same recording multiple times

An AcoustID lookup operation can return multiple results. Each of these results can contain multiple MusicBrainz recordings. However, a result can contain recordings that are also part of an other result.

As a grilo query operation returns each of these recordings, it means that the same recording can be returned multiple times. In other words, a query operation can return duplicates.

Fix the issue by storing the recordings that have already been found and checking that a result does not contain recordings that have already been returned.