Deadly Look-A-Likes

When Willow and Anya perform a temporal fold in 'Doppleganglander', they bring back vampire Willow from Bizzaroland. When the Buffy, Xander and Angel see her, they all believe that their friend is dead, until they see her perfectly living at the library. Vampire Willow decides to take over the Bronze and is surprised to hear that she has a double and decided to find her. When she does, she considers vampire her, liking the thought of having two Willows, but she ends up looked in the bookcage when the real Willow fights back.
The gang force Anya to recast the temporal fold and send vamp Willow home, just as Oz stakes her.

In 'The Replacement', a demon plots to split Buffy in two bodies, one with the Slayer strength, and the other being just a normal girl, and kill the second since the bodies cannot live alone. But he accidently hits Xander without anyone noticing. One Xander is superconfident and succeeds in everything he does, as the other gets more and more depressed as he's seriously out of luck...
Please, lady. That is so not me. He's too clean for one thing, and his socks are all matchy.'
    Xander.

The gang figures out what happened and catches them both as they prepare to reunite him, ignoring Anya's protests that it might be interesting to have two of them.

Anya: What if it doesn't work?
Xanders: Kill us both, Spock!
Buffy: They're... kinda the same now!
Giles: Yes, eh's clearly a bad influence on himself.'

Buffy is the last one to get herself a twin, but this time it's a robot, made by Warren on Spike's demand.
Buffybot: Say, look at you. You're just like me! We're very pretty!
Willow: Two of them.
Xander: Hey, I know this. They're both Buffy!
Buffy: No, she's a robot. She acts just like that girlfriend-bot that Warren guy made.

The gang manages to get hold of her and keep her unplugged at the Magic Box until they need her. The first time it's when they fight Glory, to create a diversion, and Buffybot ends up loosing its head. Willow manages to repair it and uses it to replace Buffy as the gang prepares to bring her back from the dead.