I wish I could clone myself sometimes...

I wish I could clone myself sometimes...if I could, I'd be so happy that I'd be beside myself!

This joke was pointed out to me by my architect. I am not quite sure what it is at first glance, but it is humorous, so I will analyse it.

The humor lies in the fact that the statement "I'd be beside myself" is a colloquial phrase that means "I would be very happy". In this joke the phrase could also be taken literally to mean that he would be beside a clone of himself.

I suppose that the explanation of the humor makes this a pun, but it is not a pun on a word, but on a phrase. This is moderately unusual (I can't come up with another example of such a pun off of the top of my head).