So, let's say you're on your front lawn and another player comes along to enter his house (which is the same house as yours), will you kill him/her?
Going by your story, we will both have separate flats regardless if they are the same exact one. When you are inside your own property online, it is like another world to yourself separate from the outside world. No one will be able to "enter" your room unless they are invited by you.
So if all 16 players in the online world have bought the same flat, they still will be the only ones in their flat regardless of the fact that 16 people own the same one.
Does that make sense?
My source: TypicalGamer on Youtube. He has a lot of GTA5 videos and does Q&A's which have answered this question.
But what if we both wanted to throw a party in the garden?
What about outside areas like a pool?
You can only buy apartments and no physical buildings (like Michaels house that could only exist once). A guy went around the map using the blueprint map you get in the collectors edition (I think) and every safehouse was apparently an apartment complex (he took like 35 pictures, all buildings were apartment complexes, not a single house).
I hope not as i have seen a lot of nice houses for sale which are on their own and people don't want appartments, i would prefer places like michaels or franklins