Double or Nothing is a "power-up" item in the Lingot Store.
Description
Maintaining your overall streak for seven days in a row after purchasing Double or Nothing rewards you with twice the purchase price of the item. If you have set a goal, you must reach your daily goal each of the seven days.
Store description: Attempt to double your five lingot wager by maintaining a seven day streak[1].
Purpose
This power-up serves to motivate users to keep using Duolingo consistently, and rewards them for their dedication with lingots.