This seems to work along with addweapon in cooperative mode. Once addweapon command is used in an actor, ifgotweaponce will return a value of zero whenever used in an actor of the same picnum.
The test always fails (i.e. "returns false") in single player mode.
useractor notenemy FIRSTGUNSPRITE ifgotweaponce 0 break addweapon PISTOL_WEAPON PISTOLAMOUNT enda