The Grim Adventures of Billy & Mandy: Interdimensional Misleading
Billy is replaced by "Bobby" after having a falling out with Mandy. While initially behaving like Billy, Bobby reveals himself to be a malicious usurper of other people's lives. He appears to Billy and threatens to get rid of him forever unless he disappears in 24 hours. Billy comes up with a strategy to get rid of Bobby, instead, and tricks him into getting pushed into an interdimensional portal.