Andy Weir. Just a short story.
Available online. 5471 characters long, or 2.8 standard pages.
On November 3rd 2012, someone mentioned The Last Answer on Reddit and also linked to this story, said to be similar. I read it there and then.
Fun, I guess, but not intellectually mindblowing! Notable in being about the premise Darren has expressed some interest in - that he is everyone.