10 - Worlds of the Imperium by Keith Laumer

In Worlds of the Imperium, Brion Bayards life takes an unexpected turn when he stumbles upon an alternate reality where history diverged from our own. His abduction leads him to shocking truths about a world filled with doppelgängers of notorious figures, including a tyrant bearing his own name. Tasked with an audacious mission to infiltrate the enemy stronghold, Brion must don the guise of his malevolent double, assassinate him, and assume his identity to restore order. Yet, after encountering his mirror-image counterpart, Brion grapples with the moral dilemma of killing a man who shares his face—and the dangerous forces that will do anything to prevent him from succeeding.