Double Cherries are collectible items and a power up in the Mario series.
In Super Mario Bros. 2 USA
However it should be noted, in Super Mario Bros. 2 they were just called Cherries.
In Super Mario 3D World
The Double Cherries were redesigned gameplay-wise, which will grant the player another copy of the character they are using. The player will keep making copies of that character until they touch the flagpole, or get damaged and lose one copy. When the player has copies, they control them all at once, so some strategy is needed if the players want to keep them alive. The first level the Double Cherries appear in Super Mario 3D World, is Double Cherry Pass. Bowser, or rather Meowser, also uses a Double Cherry during the second half of the fight against him.