Simpsons Season 8, Episode 1: Treehouse of Horror VII

A Halloween special which is divided into three short stories:

The Thing and I – Bart discovers he has an evil twin, who is living in the Simpsons' attic.
The Genesis Tub – After intending to prove that sugary drinks will rot teeth, Lisa creates her own miniature universe.
Citizen Kang – Kang & Kodos impersonate presidential candidates Bill Clinton and Bob Dole in order to invade Earth.