To be honest it doesn't really matter if multiple people own the same house. This isn't a role playing game and they make little effort to add an immersion factor. It would be just like your apartment, an instance where if you go in, it's your own interior. Would be nice to have options too. Just say the houses are 'similar' or 'close together' have multiple options so the houses are theoretically down the same street, despite being different variations of the same instance. Easy sh*t.
When this game come out all of my friends thought there was a mansion in the game, but there isn't though. Would be good to own one. Like the other poster said, let's get to own a second garage which range the same as the others 2-10 cars depending on the cost.
I'd like to have the option to live in a trailer in Sandy Shores as well, or have an apartment in Paleto Bay. That would be much better, living in Los Santos is getting boring and a complete change of scenery to Blaine county would be greatly appreciated. Maybe condos too? That could work instead of mansions because it's the same concept. Just houses within a block of flats. Same thing really.