Double Down is a mythic Enchantment card from Outlaws of Thunder Junction (OTJ), collector number 44. Current market price: $1.02 (foil $0.91), 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.)