As others have said above, if it's found during the hand, the hand is voided and all monies returned (and it is then replayed, same button position).
If the dupe card was in someone's folded hand, it's hard to figure how that would be discovered during the hand, because no one should be going through the muck mid-hand. But if someone was certain they had it, I guess after the hand was over the floor could be called to look. As long as it's before the next hand starts, it should void the hand. Ideally before the pot is pushed and stacked into the winner's other money, but even if not you do your best to reconstruct.
A player who knowingly plays a card that is duplicated can face further penalties, though in practice if they just say "I didn't notice" it's very hard to prove they knowingly did so.
RRoP relevant rules:
Quote:
If a card with a different color back appears during a hand, all action is void and all chips in the pot are returned to the respective bettors. If a card with a different color back is discovered in the stub, all action stands.
If two cards of the same rank and suit are found, all action is void, and all chips in the pot are returned to the players who wagered them (subject to next rule).
A player who knows the deck is defective has an obligation to point this out. If such a player instead tries to win a pot by taking aggressive action (trying for a freeroll), the player may lose the right to a refund, and the chips may be required to stay in the pot for the next deal.
A card discovered faceup in the deck (boxed card) will be treated as a meaningless scrap of paper. A card being treated as a scrap of paper will be replaced by the next card below it in the deck, except when the next card has already been dealt facedown to another player and mixed in with other downcards. In that case, the card that was faceup in the deck will be replaced after all other cards are dealt for that round.
A joker that appears in a game where it is not used is treated as a scrap of paper. Discovery of a joker does not cause a misdeal. If the joker is discovered before a player acts on his or her hand, it is replaced as in the previous rule. If the player does not call attention to the joker before acting, then the player has a dead hand.
If you play a hand without looking at all of your cards, you assume the liability of having an irregular card or an improper joker.
One or more cards missing from the deck does not invalidate the results of a hand.
TDA is more succinct:
Quote:
E: Fouled decks will be as defined by local gaming regulations and house policy. If a fouled deck is discovered, regardless of SA, play will stop and all bets will be returned. Once a hand concludes, the right to dispute based on a fouled deck ends according to Rule 22.