Double Down is a mythic Enchantment card from Outlaws of Thunder Junction Promos (POTJ), collector number 44p. Current market price: $0.98 (foil $1.06), TCGplayer market prices via Scryfall.
Whenever you cast an outlaw spell, copy that spell. (Assassins, Mercenaries, Pirates, Rogues, and Warlocks are outlaws. Copies of permanent spells become tokens.)