This might be Today we choose faces, by Roger Zelazny.
Angelo di Negri, mafia man from the 1970s, is revived a few centuries in the future by his descendants. They control a business empire descended from his mafia empire, but now mostly legitimate. They want to kill someone, but in this far future they cannot find someone willing to be a murderer, which is why they decide to revive their ancestor.
He flies a spaceship towards the victim's planet. Surprisingly, the victim is the only person on that planet; apparently he was a leader on that planet and decided to send everyone away. After killing this man, Angelo di Negri follows the victim's advice and explores the planet. He finds awesome technology on that planet. He clones himself and, more importantly, he starts erasing his own memory to make himself a better man.
At this point there is a big ellipsis. The story picks up with a bunch of clones of Angelo di Negri, evolving in a futuristic society which they mostly shaped themselves by living extremely-long lives. They don't remember him, and have several layers of locked memories in their brain, as they kept erasing the darkest parts of themselves throughout the years. A mysterious adversary is killing the clones one by one, and they are faced with a dilemma: should they unlock their memories to be able to defeat their enemy? How much should they unlock? Will they lose their sense of identity if they unlock too much? And who is this mysterious adversary?
The fact that he had to come face to face with himself is not really a plot twist, but rather a climax after a long exploration of the concepts of identity, memory, and self throughout the book.