Mirror Image

After sacrificing herself to kill Voldemort, Hayden wakes up in a limbo, in front of a mirror. When she touches its surface, she appears in a different world as Hayden Summers - the Slayer's twin sister - immediately after Buffy dies for a minute at the hands of the Master when she's called as a Slayer. With two Slayers as sisters, how will the story change? And what about Spike?