I really apriciate everyone's input on this situation. Carrie hit the nail on the head...I feel like a "bad dog owner" when Duncan chases and tries to "kill" the cats. He's a great dog, gentle with children and at all the right times but....
Anyway, Carrie, you are also right about the off-leash thing. Duncan is a big dog and very energetic. When I first got him, the people at the shelter were concerned because I don't really have a yard to speak of ( I do, but my roommate's golden has overtaken it and won't share! Duncan won't even poop out there!) So back to the point, Duncan and I go for a walk every morning up this back road that is closed to traffic and I let him run free. He chases squirrels, chipmunks, rabbits and the occasional cat from the neighborhood right up a tree... Duncan will come, sit, stay, lie down on command even from a distance but these animals that run from him seem to make him lose all control!! He'll come back,once they are up a tree..... I guess I need to work with him some more off leash....