The brother (SPOILERS)
I'm glad Daniel discovered what was going on and avoided being killed by his doppelgänger. I think between his Asian neighbor's behavior and seeing his girlfriend acting so weird, he figured it out. And seeing Gina at the end clinched it. But the question is, will he survive?
I think the fact he knows he has a doppelgänger out there means he'll be prepared for it. And that leads me to wonder if having a physical fight with an exact copy of yourself would end in a draw? Or would the doppelgänger have the advantage because it is emotionless? Or maybe you would have the advantage because you do have emotions/adrenaline?
The idea of encountering an exact duplicate of myself is more creepy/frightening than anything else I can think of.
Trivia: Rod Serling claimed to have gotten the idea for the Twilight Zone episode, "Mirror Image" following an encounter at an airport. Serling noticed a man at the other side of the terminal who wore the same clothes and carried the same suitcase as himself; Serling considered what would happen if the man turned around and was revealed to be a duplicate of himself.