Shadow Worlds — Darrell Bain

Genetic duplicates of people start dropping out of thin air right beside the originals. The duplicates are complete except for one little detail: they're dead. Frank Winston begins an investigation that widens to include the mystery of a vanished airliner that had his parents on board, as well as those of the first person who found a dead twin on her doorstep, Linda Vesprie. Together, they discover that quantum physics is even stranger than scientists had believed, and that their world and an infinite series of alternate earths are in dire peril.