A Double Mushroom is an item that is normally seen as part of a Triple Mushroom. It appears as a seperate item in Mario Kart: Double Dash!!, Mario Kart DS, and Mario Kart Wii. It allows you to use two Mushrooms at once.
This item appears in first appears as a seperate 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.
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.
This item reappears again in Mario Kart Wii. This is so far the only time where it appears as it's own item and makes no appearance in Time Trial. This item can be gotten from 4th to 10th.