Duke is the single most common rank in the Goetia's catalogue of seventy-two spirits, assigned to a larger share of the list than any other title, King, Prince, Marquis, Earl, President or Knight included. The rank name itself descends from the Latin dux, meaning leader or commander, originally a Roman and then late antique military title for a regional or frontier commander before it hardened, across medieval Europe, into a hereditary aristocratic rank just below a prince, and it is this later feudal sense, a great nobleman commanding forces and territory but still subordinate to a king or prince, that the grimoire tradition borrowed wholesale for its own catalogue of spirits.
Because Dukes are so numerous in the list, the rank functions less as a mark of any single spirit's particular importance than as the default or baseline title for a demon of substantial but not supreme standing, commanding anywhere from a modest number of legions up to several dozen depending on the individual entry, a considerably wider range of stated military strength than the more tightly defined Kings or the lower Knights show among themselves. Well known Dukes in the catalogue include Agares, said to teach languages and cause earthquakes, and Vepar, credited with governing the waters and guiding ships, both illustrating how varied a single rank's stated powers could be despite sharing one feudal title.
Sources Lemegeton Clavicula Salomonis (The Lesser Key of Solomon)Anonymous (unidentified compiler) with Encyclopaedia Britannica (online academic reference)Encyclopaedia Britannica editors