Im using the iron mace and sword, tried to restart and nothing changed. Still 2 of them and no damage. I'll try to use admin commands and clear them. Thanks yall!

It is possible to spawn more than one, if you have double the summon materials and happen to hit it twice. But not taking damage shouldn't happen. You are using a blunt weapon, right? Bonemass is resistant to everything else. Arrows might look like not much is happening. No damage at all would definitely be a glitch.

Yeah 2x boss should not be a problem. Check if you are not attacking from too far out maybe. Try to relog (if its dedi server, restart it).

Otherwise i'd use admin commands. There is one to kill all creatures around and one to clear any drops in range, this should pretty much reset the fight. You can spawn additional bone if you need it too.