Cosmic Saboteur by Frank M. Robinson

He was two men, after the meeting with the girl, Stanley Martin, the loyal Thuscan agent who continued to mastermind the betrayal of a world.

And Stanley Martin, the man who wondered at and was repelled by his own action. The man to whom the city of Chicago was strangely familiar. The man who distrusted Tanner and who knew there was a reason for it. The man in whose mind small bits of memory kept bobbing to the surface, like a ship that was breaking up beneath the sea and planks and spars kept rising to the top.