After several years away I've come back to The Simpsons around S20. I have noticed on at least two separate occasions that longer 'couch gags' have been repeated on more than one episode (one featured them breaking into a museum to find their couch, another featured them chasing the couch around the world).
I thought the whole point of the couch gag was that it was unique, a bonus scene that also pads the show length depending if the main episode is longer or shorter.
When did they start repeating gags and is it documented which have been used more than once?