Duncan's primary research interests are in the history of modern political thought and contemporary (international) political theory. More specifically, he works on the following topics:

History of Political Thought

British social and political thought, c.1850-present; modern British and American ideologies of empire and imperial expansion; and the history of the human sciences (especially political science and international relations) in the twentieth century.

Contemporary Political Theory

Contemporary normative theories of global politics; critical theory (broadly understood); varieties of political realism; the nature, scope and foundations of political thought, in particular the relationship between history and theory; aesthetics and political thought, with a focus on architecture; and conceptions of social memory and identity.

In 2012 he was awarded the Philip Leverhulme Prize for his work on the history of political thought.