U.S. Naval Base Subic Bay
Duncan was promoted to flag rank in the summer of 1958, and concurrently assigned Commander, Amphibious Group One from 1958 to 1959, followed by Commander, Amphibious Training Command, Pacific Fleet from 1959 to 1961. He took command of U.S. Naval Base Subic Bay in January 1961, During his tenure he served as president of a Philippine charity and vice president of the Philippines Tubercular Association.