Duncan began training with Inter Milan in the summer 2010. But due to FIFA regulations, he formally joined Inter in March 2011, after his 18th birthday.[2] During that time, he lived in Inter training centre "Centro Sportivo Giacinto Facchetti" in Milan.[2] Duncan made his Serie A debut for Inter in August 2012.
Sampdoria
On 19 July 2014, Duncan joined fellow Serie A side Sampdoria from Inter Milan for two seasons, until 30 June 2016.[3] Duncan moved permanently in January 2015 with Sampdoria paying a transfer fee of about €3 million[4][5] and Duncan signing a 4+1⁄2-year contract.[6]
Sassuolo
On 23 July 2015, Duncan was signed by Sassuolo on a temporary deal, with an obligation to sign him outright at the end of season from Sampdoria, for about €6 million.[5][7][8] On 6 March 2016, Duncan scored his first goal for the club against AC Milan in a 2–0 win.
Fiorentina
On 31 January 2020, Duncan joined Fiorentina on loan with an obligation to buy.[9]
Loan to Cagliari
On 17 January 2021, Duncan joined Cagliari on loan until 30 June 2021 with an option to buy.[10]
Venezia
On 27 July 2024, following the end of his contract with Fiorentina, Duncan joined newly-promoted Serie A club Venezia on a two-year contract.[11]