Um, I'm not really sure what Duncan is but GSD/Setter is as good a guess as any!
I do think he has setter in him from his coloring and his build. He's tall and slim. And his back legs sometimes look GSD to me too. But I adopted him from the SPCA and they really had no idea either as he was found roaming the streets at 6 months old.

People often guess afghan hound or flat-coated retreiver too. Whatever he is, I rarely see any other dogs that look like him. There are one or two here at Pet Talk and I have seen one in "real life" but that's about it.