DLFlux, that was a little ... confusing, but that might be a good explanation. However, I've come up with a new theory. What if the Doctor wrote a diary of his travels in the past and left it someplace by accident? Then this other "Doctor" could have picked it up, become extremely fascinated and wanted to be him. It could be the book that he did when he was the human John Smith in the third series when he was hiding with Martha.