As iconic a movie as "Superman" is, it has some physics blunders that only the Man of Steel could justify. The infamous "spin the Earth backwards to reverse time" trick stands out as one of the most egregious errors in an otherwise notable film.
The idea that one can travel backwards in time if traveling faster than light is an erroneous extrapolation made by people who misunderstand the theory of relativity. Although it is true that time slows from one's own perspective as velocity in space increases, traveling past the speed of light is impossible. The speed of light is unattainable by anything with mass, because the energy required to accelerate becomes infinite. As a professor at Virginia Polytechnic University explains, the speed of light is the same in all frames of reference.
Putting that issue aside, Superman would probably have faced a double of himself if he went back in time to stop the nuclear missiles from striking the San Andreas Fault. Of course, time is a tricky thing. If Superman actually did reverse time, he could have caused what is known as the grandfather paradox, in which going back to interfere in the past eliminates the need for the time travel in the first place, which in turn causes the time travel not to occur, which in turn requires time travel, and so on.