Not really ... which version of Envers? Is it reproducible with a test?
It´s version 1.2.2. The problem misteriously vanished after a full database recreate. Do you think it might have something to do with corrupted data? One-to-one relationships that were actually one-to-many due to lack of unique constraints.
It must have been ... a duplicate record somewhere - that's what the exception would suggest.
Next time maybe try to look for records with duplicate ids.