At least five of the characters formally have dual identities: Rupert!Ripper, Angel!Angelus, Anya!Anyanka, Willow!AltWillow and Xander!AltXander (Wishverse).

I started out to say that Giles and Angel were the only ones who shared this characteristic then realized there are more--as well as Vamps in and out of Game Face, Oz and Veruca wolf and unwolf and Hyena!Xander. But it's still something they share.