Insidious(the movie) Question!?

Lots of people assume that, but the body isn't the only thing there's two of in this situation. The astral body is just a vehicle of consciousness, not the consciousness itself. You have an astral consciousness that "drives" the astral body, and while it's doing that, your physical consciousness stays in residence in the physical body the whole time - that parking space is taken, no room for demons. Possession only becomes a realistic possibility if you actively offer permission or your physical vehicle gets torched. The usual way for this to happen is through substance abuse.