Well, it's a "what if" movie, and, if rationality were used here, a second Earth with the same people as the first Earth on it suddenly appearing in the sky wouldn't ever happen anyways.
Edit: Also the possibility that they'll explain it scientifically in the movie, might be interesting to see what they come up with.
Also, I don't want to see it to bad. I have a feeling that it will just be a love story with a slight sci-fi thing going on in the background. Kind of like how Twilight is.
