Sorcery, 4R (5) Choose another player. Double Deal deals 3 damage to that player now and deals an additional 3 damage to the player at the beginning of the next game with the player. "I'm facing defeat, ..."
Illus. Edward P. Beard, Jr.
If the next game with the player isn't a game of Magic, you still follow the instructions if you can. [Barclay 1998/08/13]
"Another player" means "a player that isn't you". One of your teammates is a legal target. A player in a different game is not a legal target. [Barclay 1998/08/13]
If played in a tournament, affects the next game in the current match only. If played outside a tournament, affects the next non-tournament game. [QAS 1998/09/09]
The term "beginning of the next game" means it triggers, after both players draw their hands (and do any mulligans), and right before starting the first player's turn, but the trigger isn't placed on the stack until the start of the first player's upkeep. [D'Angelo 2000/03/09]
A subgame does count as the next game played. [Unhinged FAQ 2004/11/15]
All tournament formats have banned cards from Unglued.
Unglued
(Common)