Wikipedia
"Doppelgangland" is the sixteenth episode of the third season of the fantasy television series Buffy the Vampire Slayer (1997–2003). It was written and directed by the show's creator, Joss Whedon, and originally aired on The WB Television Network in the United States on February 23, 1999. The episode's title is derived from the term " Doppelgänger", a German word for a look-alike or double of a living person.
The premise of Buffy the Vampire Slayer is that an adolescent girl, Buffy Summers, is chosen by mystical forces and given superhuman powers to kill vampires, demons, and other evil creatures in the fictional town of Sunnydale. She is supported by a close circle of family and friends, nicknamed the Scooby Gang. "Doppelgangland" revisits the alternate reality created in the ninth episode of the third season, " The Wish", in which Buffy never arrived in Sunnydale and vampires ruled the city. In "Doppelgangland", when Anya, a vengeance demon who previously granted the wish, attempts a spell to regain her powers. The spell fails and pulls the alternate universe's Willow ( Alyson Hannigan)—an evil vampire—into the current world and inevitably face-to-face with her human counterpart.
"Doppelgangland" primarily focuses on the character Willow, much like the season three episode " The Zeppo" focuses on the character Xander. The episode was highly praised by the media and critics upon its airing, and it is widely considered to be one of the best episodes in the series.