Doppelganger — N.R. Wick

When Duncan and his twin sister, Gwen, buy an authentic spirit board from their local Pagan supply shop, they take it home expecting to have some fun scaring their friend, Heather, by conspiring with spirits. After the pointer starts to move on its own and tells them to head to the nearest mirror to help a lost soul, the three teens find themselves haunted by a terrible paranormal force. To make matters worse, Heather hasn't been the same since. Duncan and Gwen must discover what happened to their friend and how to banish the spirit before its too late.

Doppelganger is a Young Adult short story from the collection Dark Ascension: A Demon Anthology.