The Simpsons

That was “Treehouse of Horror VII” season 8, episode 1.

http://uk.imdb.com/title/tt0779678/

treehouse of horror part 7

The Thing and I

Bart and Lisa begin to hear strange noises and think that there is something in the attic. Homer claims not to know what they are talking about, although he is later seen grabbing a bucket of fish heads and heading into the attic. Bart and Lisa investigate and find out that there really is a monster. Homer and Marge go into the attic and Homer says “oh no, it escaped”. Marge immediately calls Dr. Hibbert and he explains that Bart had a conjoined twin brother named Hugo. The two were separated at birth, but it was discovered that Hugo was too evil to live in society, so the Simpsons did the “only humane thing to do”: Hugo was chained in the attic, where Homer fed him a platter of fish-heads once a week. The rest of the family leaves to find Hugo, leaving Bart behind, but he soon discovers that Hugo never left the house. Hugo takes Bart up to the attic and ties Bart up so that he can reattach himself. Suddenly, Dr. Hibbert finds them and knocks out Hugo, commenting that the evil twin is always on the left side. But then he realizes that Hugo’s scar is on the wrong side and that Bart is the evil twin. In the end, Dr. Hibbert and the Simpson family sits down to a turkey dinner with Hugo, leaving Bart locked up in the attic eating fish heads.