Over the Horizon — Kaye Walton

They were identical twins but they never knew each other

Now one had disappeared, and the other, Gail Montgomery, was persuaded by government agents to pose as her missing sister, Monica, in the interest of national security.

Once Gail took her sister's place on the top-secret project, team leader Alex Shepard found himself suddenly attracted to "Monica." Yet he'd known her for years, and she was engaged to his best friend.

And Gail was hiding her own feelings for Alex. For her twin's sake, she had to keep up appearances with Monica's fiance. Would at, of them ever forgive her once they found out the truth?