Duke is the largest rank in the catalogue by a wide margin. Twenty three of the seventy-two spirits hold it, close to a third of the book, and between them they command 752 legions, which is more than the nine kings command together.
A category that absorbs a third of a taxonomy is usually the default rather than a description, and Duke reads that way here. The kings are marked out by precedence, the presidents by a civil and teaching function, the marquises by their place between the two, and the single knight by being an outlier. Duke is what a spirit is called when the compiler had nothing more specific to say about its standing, and the range of offices filed under it is correspondingly wide.
The legion counts support the reading. They run from 10 at the lowest to 60 at the highest, the broadest spread of any rank apart from King, and the average of roughly 33 sits close to the middle of the whole catalogue. There is no characteristic Duke.
The twenty three include Agares, second in the book, and run through Barbatos, Gusion, Eligos, Zepar, Bathin, Sallos, Aim, Bune, Berith, Astaroth, Focalor, Vepar, Uvall, Crocell, Alloces, Murmur, Gremory, Vapula, Haures, Amdusias and Dantalion, ending with a spirit credited with knowing the thoughts of all people.
Murmur is given in the Ars Goetia as a Duke and an Earl together. This collection files each spirit under a single rank, so compound titles are represented by the first of the pair.