story identification - A novel about a superhuman government assassin clone who goes rogue and tries to kill his twin's family

Plot

A super-secret government super-soldier hitman, grown in a lab with superhuman strength and regeneration powers and programmed from birth¹ to be a mindless killer, goes rogue after watching some porn or something.

He has some psychic link compelling him to pursue the man he is cloned from, an ordinary civilian citizen due to a mixup. He is like a psychotic evil twin of that citizen.

Due to psychic connection causing unknown terror, the citizen flees across the country with his family. He also had an irrational urge to flee to the West.

After a pursuit across the country, they meet in California, maybe. After recovering from complete cranial destruction by gunshot wounds by eating 20 McDonald's Happy Meals, the assassin tries to kill the whole family. The assassin's government handlers try to handle things.

Chronology and other relevant info

I would guess the novel was from the 90s. At least I think that's when I read it. I thought it was someone well-known, Koontz, Crichton etc. However I couldn't identify it from any list of their published works.

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_{1. He could possibly be a clone-born.}

Answer

Mr. Murder by Dean R. Koontz.

For author Martin Stillwater, life couldn't be more perfect. He and Paige are happy together; their two daughters, Emily and Charlotte, are intelligent and healthy; and his novels are achieving long-hoped-for levels of success. So why does he feel such dread, experience sudden blackouts?

The killer doesn't know his own name, only the name he uses. He has no family, no friends, no home. He cannot recall who gives him his assignments, and he doesn't know why his targets must die. Now, however, he senses that in a town he can't yet envision, a life awaits him, a place to call home, with family and friends.

Charlotte knows something is wrong: the man moves and sounds like Daddy, but he doesn't smile as quickly or as often as usual. And when he does smile, he seems to be pretending. Daddy isn't Daddy.