Joe, your Ambrose Duncan is out of my family. We have a Duncan Y-DNA participant in our group from your Ambrose Duncan side of family, so confirmed via Y-DNA and records.
My info is he is the son of Isaac Duncan, brother to John Duncan (my line), There father was yet another Ambrose Harrison Duncan (b.c. 1745 d. 1828 Granville Co, NC. This Ambrose Duncan was living with widow "Mary" in 1768 and the only Duncan male dropping off tax list immediately prior was a John Duncan. Living nearby in this time frame was a Peter Duncan and not far away was a Blanchflower Duncan Sr.So, I personally believe this John (wife Mary) Duncan was the John and Mary Duncan who sold out to the Rochester family in Westmoreland Co Va. in 1748. If so, (via other records) he was the son of Peter, son of John, son of Peter Duncan (b.c. 1633) of Westmoreland Co Va. Also, to add weight to this theory, Blanchflower Duncan Sr. is a proven son of Henry, son of Peter (b.c. 1633) This Peter Had lands adjoining Richard Lee, many times grandfather to Robert E. Lee. Another adjoining land owner was Robert "King" Carter Carter. Nearby was John Washington, grandfather to George....Much history in this area !
Also, many Westmoreland folks are showing up at same location of our ancestor,on Smith's Creek ( then Bute Co, NC) at the same time as the Duncans. and thissame Rochester family are in the area by 1766. Many other families living within a few miles of this old Peter Duncan of Westmoreland are also neighbors in Bute Co, NC., including the Lamkins, Cox, Rust, Harwick, Walker, Self, Dozier, Thornton, etc., etc.. Another person on Smith's Creek area was Benjamin Harrison, son of George Harison & Ann Rust, Son of George Harrison & Elizabeth Duncan (dt. of Peter b.c. 1633). Just seems to be overwhelming evidence that our Duncans are out of Westmoreland Co, Va.
Incidentally, I have a picture of your Ambrose's brother, Isaac Thomas Duncan, who served all four years of the CW ( a Robert E. Lee look-alike I might add).
If you would like an electronic copy, send me an email and I'll shoot it too you. Also, on the back of the photo is a listing of all his children and location of his grave site.
PS: Incidentally, I believe I'm correct in saying that your Ambrose Duncan's grave site is not far from the location of my GGgrandfather gravesite, Henry Harrison Duncan, son of John, son of this older Ambrose Harrison Duncan.