In 1979, Duncan helped organize Advocating Change Together (ACT), the first self-advocacy group in the U.S. for people with developmental disabilities and subsequently served as executive director of the Minnesota Jobs with Peace Campaign and the Minnesota Alliance for Progressive Action. His volunteer activities have included service as a peacekeeper on the border of Nicaragua during the Contra war, delivering medical supplies to Iraq, and working for the election of US Senator Paul Wellstone in 1990.[2]
Duncan met peace advocate David Hartsough at the Hague Appeal for Peace in May 1999. The two began laying the groundwork for an international non-governmental organization to put nonviolent strategies into practice for the prevention and mitigation of violence in conflict zones. Based on Mohandas Gandhi's concept of a Shanti Sena or "peace army," the model involves the use of trained, unarmed civilian peacekeepers working with local groups to create space for nonviolent conflict resolution.[3]