If you do not know what to do then you need to be taught. Check into an obedience school that deals with aggressive behaviors or hire a behaviorist to teach you. This is not a thing you want to play around with if you want to keep Duncan safe. If he does bite someone and they report it then you may lose him to animal control. I know this is not what you want to hear but you need to learn to be the alpha over Duncan, or else!

I would be curious to hear what you did to correct the situation with the two girls.