A duel was a type of ritualised combat.
After Baldassare Nicolotti was accidentally killed by Galileo Galilei, his father Tomasso sought revenge. When he met Steven Taylor, he mistook him for Galileo and challenged him to a duel. Christopher Marlowe took his place. Nicolotti killed Marlowe, and felt honour had been satisfied. (PROSE: The Empire of Glass)
Nicky Yusupov was killed in a duel by Baron Maneufel in 1908 Russia. At the heart of their dispute was the love of a woman, Countess Marina Heyden. His brother, Felix Yusupov, remembered the event vividly and years later shared it with Liz Shaw. (PROSE: The Wages of Sin)
In 33rd century Europa, Bad Byron and Dangerous Byron duelled to the death for the right to be Europa's only Byron. When Dangerous Byron won, Bad Byron's death activated a House of Usher mechanism, causing the entire Villa Diodati to sink under the ground. (PROSE: Managra)
Shortly after his regeneration, the Tenth Doctor had a sword duel with the Sycorax leader to make the Sycorax leave Earth. Though he lost his right hand during the fight, the Doctor was able to regrow it because it was within the first fifteen hours of his regeneration cycle. The Doctor won and made the Sycorax promise to leave humanity alone. However, as the Sycorax ship was departing, Harriet Jones had Torchwood destroy it with a salvaged alien ray weapon. (TV: The Christmas Invasion)
The Twelfth Doctor, armed only with a spoon, had a duel with Robin Hood. The Doctor defeated him using a trick where he pretended to surrender after being disarmed, then got close and tripped him into a nearby river. Robin Hood later duelled the Sheriff of Nottingham and used the same move the Doctor used on him to finally defeat the Sheriff. (TV: Robot of Sherwood)