Pastor, If any player is in possession of two identical cards, or has knowledge of two "like" cards, he can not win the pot. i.e. two aces of spades in hand, or one ace of spades in hand and another on the board. There are many situations where no players are aware of two like cards in the deck. If and when the extra card is discovered I would say that all chips would be returned to all players involved in the current hand.
I'll look through some of the rule sets and find a more definitive answer.