Prof Duncan started his academic career in 1987 and has held a number of positions at local and international institutions. In 1994 he served as the Director of the Goldstone Commission of Inquiry into the Effects of Public Violence on Children. Since 2004, he has been employed in various capacities at the University of the Witwatersrand. He started as Chair in the Discipline of Psychology and later became Chair and Head of Psychology. From 2006 to 2011 he was the Head of the School of Human and Community Development.

Prof Duncan has published widely in his field in academic journals and books. He is on the Editorial Boards of several prominent academic journals. He is currently involved in NRF funded and other research projects focusing on various social issues including child labour in the agricultural sector in South Africa and the psychological impact of HIV/Aids. Prof Duncan is a member of the Academy of Science of South Africa and President of the Psychological Society of South Africa.