Duncan is an active member of the Liberal Party in the Netherlands, the VVD (Volkspartij voor Vrijheid en Democratie).

The VVD is a party founded on liberal philosophy, traditionally being the most ardent supporter of ‘ free markets’ of all Dutch political parties, promoting fiscal responsibility, economic freedom, democracy and international cooperation, and in general committed to the idea of the welfare state. After 1971 the party became more populist, although liberal elements remain strong within the party.

From this period on the VVD became more sceptical towards the current welfare state, advocating reform of the welfare state and lower taxes in order to increase economic growth. As such it supported neo-liberal reforms to the welfare state. Often political commentators and political scientists refer to the VVD as a market liberal party in contrast to D66, which is perceived to be a more progressive liberal party.

The most important principle for the VVD has always been individual freedom.