We saw that Mels grew from a child into an adult (though she seemed to indicate that she had some control over it, at least more control than the Doctor has ever shown, or perhaps he simply doesn't care about it enough to try to control it). The original Doctor was an old man, but surely he wasn't born that way. If the Doctor we saw at the beginning really was 200 years older, that might be an indication that he's the ganger. But it's possible that he was lying about being 200 years older, for whatever reason; or its possible that he was telling the truth about the years but lying about his experiences with River.

If that's the case, then I think the group has been traveling with the ganger Doctor since the mid-season break, while the real Doctor is setting up something big (without the TARDIS). In that case, River gave her regenerations to the ganger, which could explain how he started to regenerate in Impossible Astronaut when he was shot. It could also explain why the Doctor didn't want River to give him her regenerations. Perhaps he accepted his death more easily because he's not entirely "real". Maybe in the thirty minutes when he went to change his outfit, he actually went and talked to the real Doctor about what he should do.

Hmm. I'm almost starting to like the idea.