Double Mario is a form obtained by a Double Cherry, which makes Mario gain a clone of himself.
After using the Double Cherry, Mario becomes Double Mario. This form can stack-up power-ups and even produce five clones as the maximum per player when another Double Cherry is stacked in an already activated form.
In Super Mario 3D World, collecting a Double Cherry 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. Clones will disappear if it falls down a pit, is damaged, or is off-screen for an amount of time. The clones act as protection for the player, as eventually the clones disappearing will end up with the original player, taking normal damage again.Meowser also uses a Double Cherry during the second half of the fight against him.