3x27 - The Grim Adventures of Billy and Mandy

When Billy and Mandy have a brief fight, Billy says he never wants to speak to Mandy again. The next day, Billy is over his grudge, but Mandy has already decided to find a replacement for him. Mandy introduces Bobby, the new "Billy", who turns out to be a better friend than Billy, even gaining the affection of his parents, but who also wants to take over Billy's life. A jealous Billy uses Grim's scythe to open a portal and send Bobby to another dimension.