Lame Xander: "Every Christmas, we watch Charlie Brown together, and I do the Snoopy Dance."
A character is split in two, based on personality split. Although this concept is fairly generic, the characters mention Spock to confirm that it's a Star Trek reference.
"Kill us both, Spock." gets quoted, which comes from "Whom Gods Destroy"
Xander: Not unless you want my collection of Babylon 5 commemorative plates.
Willow mentions how she handled having a twin of herself
A clip from this episode is shown.