[2] Another of Duncan's early corporate clients was the Campbell Soup Company, which hired him in 1978 to run their worldwide communications operations.

[2] Since the early 1980s, Duncan's consulting work has focused on leadership development, organizational culture, human performance, and change management issues.

His private sector clients have included IBM, American Airlines, Eli Lilly and Company, Consolidated Edison of New York, Hallmark Cards, Sprint, Black & Veatch, eBay, Texas Instruments, and many others.

In the public sector, Duncan has served the Nuclear Regulatory Commission, the Tennessee Valley Authority, the Bonneville Power Administration, the Federal Reserve Bank, and the United States Army.

[7] As a young journalist, Duncan interviewed people from a range of backgrounds, including U.S. president Lyndon Johnson,[8] comedian Jack Benny,[9] Baroness Maria von Trapp,[9] cardiac surgery pioneer Michael DeBakey,[10] historian Arnold Toynbee,[9] pollster George Gallup,[11] artist Norman Rockwell,[12] and anthropologist Margaret Mead.

One of the reporters he hired to work with him in Texarkana was Stanley R. Tiner,[7] who later would lead The Sun Herald newspaper in Biloxi-Gulfport, Mississippi, to the Pulitzer Prize.

His articles appeared in a range of newspapers, including The Christian Science Monitor, The New York Times, The National Observer, and The Denver Post.

His older brother is Stephen M. Duncan, who served as Assistant Secretary of Defense under U.S. Presidents Ronald Reagan and George H. W. Bush and is a recognized expert on national security issues.