Duncan is best remembered as the cover image on the February 1966 issue of Ramparts where he announced "I quit", as well as for his 1967 book The New Legions and his testimony to the 1967 Russell Tribunal, both of which detailed American war crimes in Vietnam.

[2] Duncan's father died when he was young, and his mother married Henry de Czanyi von Gerber, a naturalized American, cellist and orchestra conductor.

[3] Duncan transferred to U.S. Army Special Forces (the "Green Berets") in the first part of 1961, where he continued to work in the field of operations and intelligence.

[4] Duncan was deployed in Vietnam in March 1964, serving in a variety of capacities with the 5th Special Forces Group and Project DELTA, which he helped to organize.

[2] Duncan was also tapped to help write the official history of U.S. Special Forces in Vietnam, spending the last 6 or 8 weeks of his tour engaged in this task.

In 1967 Random House published a book written by Duncan entitled The New Legions which was sharply critical of the American military campaign in Vietnam while exposing many details about the Green Berets.

[6] There he detailed a de facto class in torture techniques conducted for members of the Special Forces entitled "Counter-Measures to Hostile Interrogation.

Editor William McDonald explained that the death became known to the newspaper during research on what was planned to be Duncan's advance obituary, written by Robert D. McFadden.

Regarding the decision to complete and publish the article seven years after the subject's death, McDonald said:[8] If another news organization, particularly one with national reach, had run an obituary in 2009, we would have stood down, acknowledging that we had been napping back then and that it was way too late now to make up for the lapse.