[1] In 1904, Duncan took up the general secretaryship of the Scottish Steam Vessels Enginemen's and Firemen's Union (SSVEFU); he led it through two strikes, in 1905 and 1907, but both took place against his wishes and achieved little.
This inspired him to join both the Fabian Society, and the Independent Labour Party (ILP), almost immediately being appointed as its full-time secretary for the East of Scotland.
[1] Duncan opposed World War I, but served on the Scottish Departmental Committee on Food Production; through this, he managed to obtain exemption from being conscripted.
[1] He stood for the Labour Party (to which the ILP was affiliated) at the 1919 Aberdeenshire and Kincardineshire Central by-election, taking 26.4% of the vote but only managing third place.
He championed land nationalisation, but was generally considered to be on the right-wing of the ILP, and one of James Maxton's leading opponents in the Scottish branch of the party.