The joke is a play on words.  With playing cards, one person is the 'dealer' and they 'deal' the cards out to each player by drawing and placing them before each player.  Saying 'I'll deal with you later' means 'I will take care of you later.' Since the person thinks they are a deck of cards, it could be taken to mean dealing cards out.
Doctor , doctor , I keep thinking I ' m a deck of cards . - I ' ll deal with you later . Next !