Stranger With My Face

Blurb

Stranger with My Face is a young-adult horror novel by Lois Duncan, first published in 1981. It was adapted into a television film. The book is about a girl named Laurie Stratton whose boyfriend and family members start seeing her in places where she knows she wasn't, even couldn't have been at times.
As the story progresses Laurie is contacted by the stranger and finds out "Lia" is her identical twin sister; their mother gave them up at birth and later reclaimed the unadopted Lia. Lia, tells Laurie that she can appear to her by using astral projection, and teaches her how to astrally project so they can meet. Laurie, who doesn't look anything like her family, asks her mother if she is adopted or has a twin; eventually her mother admits not only that they couldn't afford to adopt both babies, but that the other baby gave off "bad vibes," so though both the Strattons and the adoption agents didn't agree with separating twins, the Strattons took baby Laurie with them and left the other baby behind. They later learned that the biological mother had reclaimed the other twin.
Laurie becomes able to astrally project. Meanwhile, people close to her start to have accidents.