There are two cases of what must be twins in this movie..
1) When Shrek takes off on the Dragon, the Donkey is on the ground yelling at him that he hasn't had time to install seats yet. However, when Shrek and the dragon land at the castle, the Donkey is suddenly there riding in front of him? Since we never saw the donkey get on the Dragon, perhaps this is his evil twin? Hmmm.
2) In the ending wedding scene, the 3 blind mice are turned into blind horses for the carriage, however only seconds later are they seen back in mouse form dancing with the band (but wait it gets worse..) Then AGAIN they are seen as blind carriage horses. EVIL TWINS INDEED!