Le Troisieme Jumeau (J'ai Lu)

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.

From Amazon:

Using a restricted FBI database, genetic researcher Jeanie Ferrami has located identical twins born to different mothers. Frightened by her bizarre discovery, she is determined to discover the truth at any cost—until she finds herself at the center of a scandal that could ruin her career.

To extricate herself, Jeannie plunges into a maze of hidden evidence. With growing horror, she uncovers a cynical, far-reaching conspiracy involving disturbing genetic experiments and some of the most powerful men in America—men who will kill to keep their secrets concealed.