6th of Ardsheal. 10th Chief of Clan Appin. Son of Charles Stewart, 5th of Ardsheal. Married Anne Irving (1740-1802) in 1767. Duncan and Anne had ca. 10 children.
In November 1762 he was on his way to receive orders from the East Indian company. It seems that he relocated to Boston, Massachusetts a few years later. He married his wife Anne Irving in Boston in 1767, and a couple of portraits of the two (painted 1767 in Boston by John Singleton Copley) are currently in the possession of the Scottish National Portrait Gallery.
Stewart appears to have been Collector of Customs in New London, Connecticut in the mid-1770s and on Bermuda in the mid-1780s, having been back in England in between.
[see comment below by James J. Caudle]