Mirror, Mirror — Virginia Nielsen

Gilly and Gerry are identical twins, separated at birth. They meet for the first time when they're sixteen and find that they're so alike, even they're boyfriends can't tell them apart!

Then Gilly visits Gerry's home, and when the vacation is over, Gerry convinces Gilly to switch places secretly. It's fun at first, but soon strange accidents begin, and Gilly receives frightening phone calls meant for Gerry. More confusing, Gilly is falling in love with Gerry's boyfriend.

Is Gerry in some kind of trouble? Why didn't she tell Gilly? And why did she set up to be in terrible danger?