"Then we can only give thanks that this is Antarctica, where
there is not one, single, solitary, living thing for it to
imitate, except these animals in camp."

"Us," Blair giggled. "It can imitate us. Dogs can't make four
hundred miles to the sea; there's no food. There aren't any
skua gulls to imitate at this season. There aren't any
penguins this far inland. There's nothing that can reach the
sea from this point - except us. We've got brains. We can do
it. Don't you see - it's got to imitate us - it's got to be one
of us - that's the only way it can fly an airplane - fly a plane
for two hours, and rule - be - all Earth's inhabitants. A world
for the taking - if it imitates us!
[ Who Goes There?, by John W. Campbell ]

Xander: Let go! I have to kill the demon bot!
Xander Double (grabbing the gun): Anya, get out of the way.
Buffy: Xander!
Xander Double: That's all right, Buffy. I have him.
Xander: No, Buffy, I'm me. Help me!
Anya: My gun, he's got my gun.
Riley: You own a gun?
Buffy: Xander, gun holding Xander, give it to me.
Anya: Buffy, which one's real?
Xander: I am.
Xander Double: No, _I_ am.
[ Buffy the Vampire Slayer, Episode 5.03, "The Replacement" ]