i kinda like this idea....

We know that when Desmond failed to push the button, it caused the electromagnetic annomaly that inturn caused the plane to split... Now the writers have said that the island was not moved at this point but SOMETHING happened.. i believe that the plane broke off into 2 timelines or maybe points in space... the plane that they found at the bottom of the ocean is the ACTUAL plane as is the crashed one on the island... due to the elctromagnet charge release, the island or destiny (whatever force may be working) doesn't know if the pasengers are atually dead or alive.. hence there being 2 timelines now.... the "island" thinks they are alive, the "real world" thinks they are dead... they are both.. and that's why they belong on the island... it's the only place where them being alive makes sense... there are therefore in fact 2 x each O6 character who escapes the island... the ones who actually left.. and their dead corpses lying at the bottom of the ocean in the plane wrekage....