You're doing a great thing by getting training for Duncan do not be upset about that!!! If you don't train Duncan he will be truly out of control so this is what he needs.

I had an aggressive foster dog for a short time, short because submissive me and submissive Sadie were more his prisoners than caretaker and friend. I found a group in yahoo that dealt specifically with aggressive dogs. You need to go to http://groups.yahoo.com, register with yahoo or sign in if you are registered there, and search for the group "agbeh" (for aggressive behavior). You would then have to go thru a membership process that takes about a day. This is a great group that gives great tips on dealing with your dog, how to understand his behavior and adjust yours. The tips come from trainers and only positive reinforcement recommendations are permitted.

This option is free, and can add to the help a trainer in person gives you. They may even know of a trainer in your area.

Good luck, you can do it, and in the meantime take any preventive measures you can think of such as wearing a muzzle (Duncan that is) and locking the door to keep out the monster midget!