At the 2020 Tokyo Olympics, Duncan won silver in 200m freestyle, finishing 0.04 seconds behind the winner and teammate, Tom Dean. He followed it up with a gold as part of the 4×200m freestyle relay team with Tom Dean, James Guy, and Matt Richards, winning it in a European record of 6 minutes 58.58 seconds. He also claimed a silver in men's 200 metre individual medley, and another in the 4 × 100 metre medley relay with Luke Greenbank, Adam Peaty, and James Guy, making him the first British athlete to win four medals in a single Games leading to calls for him to be nominated for a knighthood.