A Double Mushroom is an item that is normally seen as part of a Triple Mushroom. It appears as a separate item in Mario Kart: Double Dash!! and Mario Kart DS. It allows you to use two Mushrooms at once.
Mario Kart: Double Dash!![]
This item appears in first appears as a separate item in Mario Kart: Double Dash!! and it gives you a speed boost 2 times unlike a regular Mushroom. This item also appears in Time Trial for this game replacing the Triple Mushroom.
Mario Kart DS[]
The item reappears in Mario Kart DS once again with the same function as it's predecessor. It can also be used in Time Trial if you use a Standard Kart.