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Your situation is pretty rare. I would be in favor of a redeal. I would base my decision on two factors: #1 The incident occurred before any action has been taken and #2 The uncertainty of the proper card to either player.
Once a card is somehow intermingled into another players hand, the player with too many cards would have no possibility of competing for the pot. I'm not quite sure if the TDA has a specific rule to cover your scenario, other than players must protect their own hand, but killing the hands is too extreme, in my opinion. Gather in the cards, reshuffle and deal again.