Star Trek (Western Publishing Co. 1967)

The Enterprise investigates the location of the Big Bang, and encounter a duplicate Kirk, who claims that the Universe repeats identically in all but a single moment of free will. His Universe had passed that moment, and the known Universe was coming up to it, which means everything up to this point had been identical in both Universes (except, it seems, for the choice of Starfleet Uniform colours.) The alternate Kirk must fight this Kirk to the death to control the choice of free will.