Here's my perspective on this:
>If you have an identical twin, is your twin "as much you" as you are yourself?
No in pretty much all meaningful senses; there was never a point where you and your identical twin had the same memories.
>At the moment of replication, which of the two is really "you" (can anyone, including you, tell?), and how will that identity develop into the future as the two have divergent experiences?
This is where the concept of personal identity itself gets tricky. I would say that in a way both of them are you. As far as my subjective experience is concerned, what makes me "me" is primarily my memories, and both instances would share that at that point in time.
This also means that, as two instances exist with diverging experiences, their "identities" diverge as well.
An outside observer might view the original and the copy differently, but personally I view subjective experience as the most important factor here.