A versatile player at 1.88 metres (6.2 ft) tall and weighing 88 kilograms (190 lb), Duncan is able to contribute in the forward line as well as the midfield.

Duncan's promising junior performances were rewarded when he received a scholarship within the prestigious AIS-AFL Academy as part of the 2008 intake.

The following season, Duncan achieved State honours for Western Australian as a bottom-aged player during the AFL National Championships.

Duncan again received mid-year State honours for Western Australia in the AFL National Championships, and was named vice-captain of the squad.

[1] His performances in the championships—during which he averaged 20 disposals at a team-high 75% efficiency, ranked third overall for total marks, and kicked eight goals[6]—earned him end-of-year All-Australian honours and helped Western Australia capture the 2009 national championship.

[8] He made his senior debut in the opening round of the 2010 AFL season, recording 10 disposals and 6 tackles against his childhood team Essendon.

[9] During a round four fixture against Port Adelaide, Duncan recorded a career–high 20 disposals and 9 marks to help Geelong achieve a 95-point victory.

[1] Kevin Sheehan has complimented Duncan on possessing "good foot skills ... on both sides of the body", while showing an "ability to read the game to advantage ... and make excellent decisions".