Mario Luigi is a flawed clone of Luigi created by DiC, who later employed him as a stunt double for Mama Luigi. As you can see in the images here, however, they occasionally failed to hide his presence. Like most Luigi forms, he did not survive the Form Wars.
Mario Luigi has brain problems due to an error in the cloning process. As a result, he is mute and almost always has a moronic, open-mouthed expression of joy on his face. However, he also possesses some kind of mental ability he uses to make other people, like Brooklyn Mario, speak for him.