Android at Arms — Andre Norton

EVIL GNAWING AT THE HEART OF EMPIRE
Was he an android? Or was he, as he thought, the rightful Emperor Inyanga, imprisoned on an alien planet and kept in mind lock while an android ruled in his place?

Even after he met his double he could not be sure, but he knew his life was worth nothing if he fell into his rival's hands. Hiding from pursuit in a remote and supposedly haunted corner of the great palace, he was summoned to a strange fire by the sound of an ancient harp and drawn into a world that mirrored his own -- but a world so beleaguered by enemies from within and without that it seemed it must perish.

Yet, somehow, he had to save it....and with it the whole Inyangan civilization.