Sounds like Duncan is making good progress, even if he still has some issues. I think the attitude of the other peeople really does make a difference.
Sadie, who is the most submissive dog, will growl and bark when we walk by one of two houses in the neighborhood, if the dogs are out. One house has two labs, the other two rotties. She doesn't do it in front of any other dogs in the neighborhood, no matter how many are in the yard, or how big or small the dogs are. I think she just gets a bad vibe from those particular dogs and feels the need to protect me and Cincy. I guess the difference with her is that once I tellher it is okay, she settles down...hopefully Duncan will get there too!!

Good luck!!!