Duncan trained with Internazionale since summer 2010. However due to FIFA regulation, he formally joined Inter in March 2011, after his 18th birthday. He lives in Inter training centre "Centro Sportivo Giacinto Facchetti" in Milan. Duncan made his Serie A debut for Inter on August 26, 2012 in a match against Pescara when he came on as a substitute in the 86th minute for teammate Walter Gargano.
On 19 July 2014, Inter agreed to loan Duncan to the fellow Serie A side Sampdoria for two seasons, until 30 June 2016.