Far Shore of Time — Frederik Pohl

Dan Dannerman has been through hell. Caught in the middle of an interstellar war that will end only with the death of the universe, he's been captured by aliens who call themselves the Beloved Leaders, cloned repeatedly, torn from his wife, and brutally tortured.

Sitting in a prison cell on an alien world, slowly going mad, Dan is finally freed by the Horch, the sword enemies of the Beloved Leaders. The time has finally come for Dannerman to strike back--but at whom?

Trusting neither side, Dannerman must somehow convince the Horch to send him back to Earth so he can warn humanity of the approaching alien menace. But when he finally returns he finds an Earth far stranger than he can possibly imagine, an Earth that already has two Dan Dannermans--an Earth already under seige by the Beloved Leaders...