This is something that both stumps and excites me. I just cannot fathom how they will allow us to buy apartments without the possibility or running into duplication of users living in the same space.
Unless of course the only way you can access your place is in a private lobby with only your friends/crew members.
A way some others have suggested is for it to work in the same way that teleporting to the top of a building to base jump from worked in previous games with a marker on the ground. If 2 people own the same space then no problem as it teleports you to your own little space. However the CVG guys have said that another player could ring your doorbell and you can use your peephole or CCTV to see who it is before letting them in.
Also we can actually see into the world through our windows, this wouldn't work in the teleporting method.