Enchantment, 3UU (5) When Duplicity comes into play, remove the top five cards of your library from the game face down. At the beginning of your upkeep, you may remove your hand from the game face down. If you do, put into your hand all other cards you own removed from the game with Duplicity. At the end of your turn, discard a card. When you lose control of Duplicity, put all cards removed from the game with Duplicity into their owners' graveyards.1
Illus. Dan Frazier
The cards are face down all the time. You can't look at them. This applies even to cards from your hand that got swapped out. You have to remember what they were if you care. [bethmo 1999/03/13]
Taking control of another player's Duplicity will not allow you to use their cards. [Jackson 2000/10/24]
Tempest
(Rare)