Duncan was born in Oakland, California, as Edward Howard Duncan Jr. His mother, Marguerite Pearl Duncan, had died in his childbirth. He was her tenth child and the delivery was at home to avoid the risks of contracting the so-called Spanish influenza at a medical facility. Duncan's father was unable to afford him, so in 1920 he was adopted by Edwin and Minnehaha Symmes, a family of devout Theosophists. They renamed him Robert Edward Symmes in honor of a family friend. It was only after a psychiatric discharge from the army in 1941 that he formed the composite of his previous names and became Robert Edward Duncan.