(covers information from several alternate timelines)
He helped recruit Seven of Nine from 2375, almost five hundred years in his past, and assigned her to a mission of identifying the individual responsible for placing a temporal disruptor aboard the USS Voyager, that resulted in the ship's destruction.
When it was later discovered that the saboteur was, in fact, a future version of Captain Braxton, he had Braxton arrested and took control of the mission. He then recruited Captain Janeway, as Seven would begin to suffer deleterious effects to her health if she were to make another time jump. Ducane sent Janeway back to 2372, where Braxton initially planted the disruptor, allowing Janeway to change history by preventing Braxton from completing his mission.
Ducane then sent both Janeway and Seven back to their proper place and time, warning them not to discuss their experiences with anyone, in accordance with the Temporal Prime Directive. (VOY: "Relativity")
His last name was spelled "Duquesne" in the All Our Yesterdays sourcebook.