Loser's Guide to Life
Imagine a man who gets physically duplicated by an alien. He has the exact same brain, same cells up there, same memories, same recollection of sensations, same personal favourites. So when he sees “his” cat, for example, he knows how “he” feels about it. And so it is for everything else. But this is not really “him”, of course.
He talks about himself using the first person singular pronoun in almost audible parentheses. “Well, 'I' guess 'I' should be going now...”, he says. “...uh, 'I' will be seeing you later. So to speak.”
Although he is completely and intimately knowledgeable about this other person's life, it's still not his life. What, if anything, does that mean? Could he still know that he wasn't that man? Would he maintain an additional self above this one? How would that work? Wouldn't it get confusing?
More importantly: What if a normal person had this sensation, without being an alien? What if he were writing on this weblog?
I assume these questions have been dealt with in any number of science fiction and fantasy works, but as far as I know, the whole thing is usually treated as some kind of bizarre thing that gets revealed towards the end. I was thinking of a story where you would take this as your starting point, because it's quite normal.