I've probably managed to find the source of this issue, it's the buildings locality of course. Building is destroyed using _building setDamage [1,false];, everything should be ok as it's AG/EG function. @veteran29 helped me with pointing out that every player has his own copy of the building with the same ID. So we thought that when executing damage function on building, the server object is damaged/destroyed and synchronized but player's building wasn't really touched (it's a different object but with the same ID, right?), that's why we have building duplication. Later we've found out that only the destruction doesn't work and remoteExec'ing the setdamage made it work on dedicated (but not in 3den).