Duke was an aristocratic title. The female form was Duchess.
Ben Jackson often called Polly Wright "Duchess" due to her upper class breeding. (TV: The War Machines)
The Mandragora Helix diverted the TARDIS to 15th century Italy, where the Fourth Doctor and Sarah Jane Smith met Duke Giuliano. (TV: The Masque of Mandragora)
Duke Walid and Duke Armand were rivals for the throne of the Earth Empire after the Divine Empress's assassination. (PROSE: So Vile a Sin)
Orsino and Estella were the Duke and Duchess of Venice in the late 22nd century. After Estella faked her death, Orsino continued to be Duke for the next century. (AUDIO: The Stones of Venice)
Torchwood India was run by Eleanor, Duchess of Melrose in the early 20th century. (AUDIO: Golden Age)