I’m looking for the title of a book I read sometime between 1984 and 1991, which had probably not been published very long before that, about twin girls who had been separated and adopted by different couples when they were very young. I think their parents had died, and the twins were split by their grandparents. One twin lived in San Francisco. (I’ll call her Twin 1.) They discovered that they were twins when one of the girls found her original birth certificate which listed her as a twin and confronted her adoptive parents. The girls met, and Twin 2 went to San Francisco to visit Twin 1. There was something spooky going on, and the San Francisco fog was played up for the creepiness factor. I believe it ended up that Twin 1 knew someone or something was trying to kill her, and when she discovered that she had a twin she planned to take over Twin 2’s life and leave Twin 2 to die in her place.
This is not Stranger With My Face by Lois Duncan.
I forgot to mention that the girls were in their teens when they discovered that they were twins, and that I believe Twin 1 needed a passport, which was the reason she saw her birth certificate.
Maybe “Mirror, Mirror” by Virginia Nielsen? It was part of the Windswept Mystery series (#17) and published in 1983.
Yes! I’m almost certain that’s the one. I’d forgotten what the cover looked like, but when I saw it I remembered exactly what it looked like. Thank you! I’d say this one is a successful solve.