For example, if a 'house' ingame was the actual house a person was using, and not just a stand-in for a location that exists in lore but not ingame due to a far smaller game world than the actual lore suggests, then poor Dernes' house gets used by like, fifty different people.
I think it depends a little on the exact location in question. (And I may be misunderstanding what you said, and this may be exactly the same thing reworded. Apologies if that's the case! :P
I'm gonna use Hearthglen as an example because it's Pia's IC home and also because it's shared with Razortalons.
Obviously, there's only one Hearthglen in the world, right? But it -- like virtually every location -- is bigger than we see in-game. So while the space of Hearthglen can't be duplicated and *could* be claimed if, y'know, it wasn't already claimed by the Argent Crusade, the buildings represented in Hearthglen can be. The house has served as a generic home, Taelanas' house, and various other people's homes and I'm sure Razortalons has used it as well. The mage tower has represented the Pia tower (which obviously doesn't exist in game) and has been used by RT as a different tower (sometimes at the same time!)
But you also have buildings like the Blue Recluse in Stormwind. While Stormwind is surely bigger than it appears, and even the Blue Recluse is probably a mere representation and probably would look totally different if it were an actual place in an actual Azeroth.... I think we're all agreed that it's a specific, unique place of which there is only one. Zaium's been the agreed-upon owner for as long as I've been on CC (unless something changed recently?). There's been a few hiccups where people new to the server have tried to set up shop and claim ownership, but they've always worked themselves out and people have generally been very respectful of that claim (as far as I know).