Mario Luigi is an unconfirmed form of Luigi. His voice is completely different compared to the other forms.
After getting transported back to Brooklyn, Luigi felt really strange until he opened his mouth. Due to a malfuction with the Mushroom Kingdom/Brooklyn Warp Pipe, Luigi has Mario's voice and behaviour but has to use Mario for his hand movement.
Later he dyed his overalls red, however he was rumoured to be defeated when Debrotherized Luigi was unleashed.
Another occurrence is when Mario and Luigi once picked up a pipe known as "The Koopa Catcher". Upon touching the pipe, Mario's voice changed into Luigi's and carried on moving his mouth once he had finished speaking, whilst Luigi was wearing a red hat.
Yet another incident was somehow triggered by being trapped in a giant birdcage when King Koopa was shrinking the prince of Giant Land. This was his third and final form. Mario and the gang could not let this form of Luigi cause any trouble, so they had to kill him.
Also, while in a forced wrestling match, he body-slammed Mario so hard the colors of their clothes switched. This also created a temporary Luigi Mario, which hasn't been seen ever since.
One suggested origin of Mario Luigi is awful animation and coloring on the part of DiC
Mario (which most of his characteristics are shown)