Shooting game help

Hey.
OK this is my problem.
i used this script.
_root.onMouseDown = function(){
_root.attachMovie(“bullet”, “bullet”, _root.getNextHighestDepth)
bullet._y += - 10
}
i use the mouse to move the symbol.
whenever i shoot when the bullet is still being shot, It starts over again.
is there any way to make it so that when you shoot… you can;t shoot again until the bullet goes off stage.
thanks
-cambo123