The whole Superman versus Superman thing has been done plenty of times before, perhaps most famously (if not brilliantly) in the movie Superman III. This collection of Superman comics, pulled from a variety of different sources in and around the Infinite Crisis event, takes a slightly different path.
This is where it gets a bit complicated. The Crisis is all about dissolving DC Comics’ various universes into a single, coherent universe. However, these universes aren’t collapsing without a struggle, and two different Supermen from two different universes must fight to save the Lois Lane they love. There can be only one winner and one universe, and all this inter-dimensional fisticuffs could bring everything crashing to an end.