Joseph Alfred Duncan is a Ghanaian footballer who plays for Cagliari, on loan from Fiorentina, as a midfielder.

Duncan began training with Inter Milan in the summer of 2010. But due to FIFA regulations, he formally joined Inter in March 2011, after his 18th birthday. During that time, he lived in Inter training center "Centro Sportivo Giacinto Facchetti" in Milan.

Duncan made his Serie A debut for Inter on 26 August 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. The club bought Duncan outright for about €3 million in January 2015. He signed a 4½-year contract.

On 23 July 2015, Duncan was signed by Sassuolo on a temporary deal, with an obligation to sign him outright at the end of the season from Sampdoria, for about €6 million. On 6 March 2016, Duncan scored his first goal for the club against A.C. Milan in a 2–0 win.

On 31 January 2020, Duncan joined Fiorentina on loan with an obligation to buy.

On 17 January 2021, Duncan joined Cagliari on loan until 30 June 2021 with an option to buy.

Duncan represented Ghana at the 2013 FIFA U-20 World Cup in Turkey playing four games during it. He made his full international debut on 14 November 2012 against Cape Verde.

Campionato Nazionale Primavera: 2011–12

NextGen series: 2011–12

FIFA U-20 World Cup: bronze medal in 2013