The Third Twin — Ken Follett

Young scientist Jeannie Ferrami discovers a baffling mystery. Two young men, law student Steve and convicted murderer Dan, appear to be identical twins. Yet they were born on different days, to different mothers. Jeannie investigates, but shadowy forces retaliate and Steve is accused of a terrible crime.
As Jeannie falls in love with him, can she be sure he is different from his evil twin? Solving the mystery will mean deadly danger.