The Replacement

In which we learn that two Xanders only doubles the fun if you are Anya, and want to have sex with both of them, or Riley, and want to experiment on them. Reverse and reorder as you see fit.

Official back-of-box synopsis:

A demon Toth tries to destroy Buffy, but is unsuccessful. Instead he splits Xander into two bodies, each with a different side of his personality. As each one tries to convince his friends that he is the real Xander, Buffy must find a way to put the Xanders back together.

From the old WB Buffy website:

NICHOLAS BRENDON'S IDENTICAL TWIN BROTHER GUEST-STARS - When a crafty demon out Buffy hunting accidentally catches Xander in his sights, a mysterious Xander double is born. Two Xanders aren't better than one as the evil double quickly takes over Xander's identity, fooling everyone including Buffy, and starts living Xander's life with a flourish.

Cross-reference:

episodes in which characters have twins:

• Doppelgangland
  Evil Vampire Willow from the Wishverse
• Intervention
  The Buffybot
• this episode
  Xander isn't himself, he's himselves...

episodes in which there's dancing:

• When She Was Bad
  Bad!Buffy has Xander all hot and bothered
• Faith, Hope And Trick
  Faith is first seen dancing
• Bad Girls
  Buffy and Faith dance subtextily together
• The Prom
  Buffy and Angel 4 EVA!!!
• Who Are You?
  Faith takes Buffy's body onto the dancefloor
• Once More, With Feeling
  Again with the "well duh!"
• Him
  Daddy like! Oh my god that's Dawn!
• this episode
  The Snoopy Dance

Cast:

• Sarah Michelle Gellar as Buffy
• Alyson Hannigan as Willow
• Nicholas Brendon as Twin Xander
  

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• Nicholas Brendon as Xander
  

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• Anthony Stewart Head as Giles
• Kristine Sutherland as Joyce
• James Marsters as Spike
• Emma Caulfield as Anya
• Marc Blucas as Riley
• Michelle Trachtenberg as Dawn
• Kelly Donovan as Xander's Body Double
• Michael Bailey Smith as Toth
• Cathy Cohen as Building Manager
• David Reivers as Foreman
• Fritz Grieve as Construction Worker