Seems one needs the Advanced Camera plugin to open this cap.
Oh well, I can only speculate at this point. If the bullet is duplicating, then it must be a duplicated trigger, event or action. It may not be always apparent; are you using two instances of Mouse and Keyboard object? Only one global instance is enough. Next check any conditions related to firing; perhaps it is allowing two triggers before it clams. Then check actions, sometimes it is not apparent but some people mistakenly duplicate the spawning event.
Moreover, check if the bullet isn't in a container with another bullet, though that sounds silly by itself - objects cannot be containers of themselves.
Just a few tips to tide you over until someone with the camera plugin can dig in.