So assuming that Mace Windu truly did defeat Darth Sidious in a lightsaber duel, would that make him the only individual to have ever done so? Yoda brought Sidious to a draw but didn't defeat him.
In canon, no one is known to have defeated Darth Sidious in a lightsaber duel.1 Presumably, Sidious' master Darth Plagueis defeated him during training, although this might not be considered a duel in the true sense of the word.
In Legends, Sidious (actually one of his clones) was defeated by Luke Skywalker at the Battle of Pinnacle Base (although Luke was aided by Leia).2 Sidious lost his left hand as a result of the duel. To the best of my knowledge, this was the only time Sidious is known to have been defeated in a lightsaber duel all of Legends.1
1 Because Sidious threw the fight against Windu, of course. :)
The reason why Mace Windu defeated Sidious was also because he had a power amplifiar. Per Star Wars Legends(which I will be using since there is practically know information on the lightsaber form/abilities of Mace Windu other than the fact that he is powerful), Vaapad users could channel an opponent's dark side energy to fuel their own, and become extremely powerful in combat.
In Disney canon, no one other than Mace defeated Sidious in combat, and it can be also argued that Sidious was faking this to trick Anakin into thinking that he was the victim.
Per the Legends, at the Battle of Pinnacle Base, Luke defeated Darth Sidious, albeit with some aid from his sister, Leia Organa.