Double Play is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Choose another player. Search your library for a basic land card, put it onto the battlefield, then shuffle. At the beginning of the first upkeep in your next game with that player, search your library for a basic land card, put it onto the battlefield, then shuffle.