To run a Duplicate Sealed event, five booster packs are opened and a list with the contents of the packs is provided to each of the players. They then use their own cards to recreate the exact same pool and build a sealed deck using regular Limited rules. Players are free to add as many basic lands (Plains, Islands, Swamps, Mountains or Forests) to their decks as they like.
Duplicate Sealed lets players see how their peers and friends would build a deck when given the exact same card pool. Will you play the same creatures as the rest of the people you play Magic with, or will they come up with a completely different concoction? Duplicate Sealed lets you find out!
Duplicate Sealed has been used many times at the Magic Invitational. When played at the Invitational, the card pools often include cards supplied by Magic R&D (especially Mark Rosewater) that are created just for the Invitational and have never been seen before by the players.