Cage Warriors In his MMA debut, Duncan faced Kyle McClurkin on September 26, 2020, at Cage Warriors 116. He won the fight via technical knockout in round two. Duncan faced Lukasz Marcinkowski on December 12, 2020, at Cage Warriors 119. He won the fight via technical knockout in round one. Duncan faced Will Currie on March 18, 2021, at Cage Warriors 120. He won the fight via technical knockout in round two. The rematched with Currie took place on June 24, 2021, at Cage Warriors 123. He won the fight via unanimous decision. Duncan was scheduled to face John-Martin Fraser on September 30, 2021, at Cage Warriors 127. However, the bout was moved to Cage Warriors 131 on December 10, 2021, for unknown reasons. In turn, Fraser pulled out and was replaced by Justin Moore. He won the fight via a rear-naked choke in round one.
Cage Warriors Middleweight Champion Duncan faced Djati Mélan for the
Cage Warriors Middleweight Championship on April 2, 2022, at Cage Warriors 136. He won the title via technical knockout in round three. In his first title defense, Duncan faced Marian Dimitrov on November 12, 2022, at Cage Warriors 146. He defended his title via technical knockout in round one. Duncan was scheduled to face UFC veteran
Jesse Taylor on December 31, 2022, at Cage Warriors 148. However, the day before the event, Taylor was removed from the event after he was involved in a hit-and-run incident.
Ultimate Fighting Championship After the bout with Taylor was cancelled, as the main card broadcast of Cage Warriors 148 got underway, Duncan revealed that he had been signed by the
Ultimate Fighting Championship and prepare for his debut at
UFC 286 in London. In his promotional debut, Duncan faced
Duško Todorović on March 18, 2023, at UFC 286. He won the fight via technical knockout after a knee injury rendered Todorović unable to continue in round one. Duncan faced
Armen Petrosyan on June 17, 2023, at
UFC on ESPN 47. He lost the fight via unanimous decision. Duncan was scheduled to face
César Almeida on November 18, 2023, at
UFC Fight Night 232. However, Almeida was pulled out from the event for medical reasons and was replaced by Denis Tiuliulin. He won the fight via technical knockout in round two. Duncan faced Cláudio Ribeiro on March 2, 2024, at
UFC Fight Night 238. He won the fight via technical knockout in round one. Duncan was scheduled to face
Robert Bryczek on July 27, 2024, at
UFC 304. However, Bryczek withdrew from the fight for unknown reasons and was replaced by
Gregory Rodrigues. He lost the fight via unanimous decision. Duncan faced Andrey Pulyaev on March 22, 2025, at
UFC Fight Night 255. He won the fight via unanimous decision. Duncan faced
Eryk Anders on August 9, 2025, at
UFC on ESPN 72. He won the fight by knockout via spinning back elbow and ground and pound near the end of the first round. This fight earned him his first
Performance of the Night award. Duncan faced Marco Tulio on November 8, 2025, at
UFC Fight Night 264. He won the fight via knockout in round two. This fight earned him another
Performance of the Night award. Duncan faced
Roman Dolidze on March 21, 2026, at
UFC Fight Night 270. He won the fight by unanimous decision. ==Championships and accomplishments==