My favorite duke is definitely the Duke of Hawkshire, James Fielding, from Anna Depalo's "One Night with Prince Charming!" I just love everything about him and the book, and it's definitely one of my go-to's for when I am looking for a good historical romance.
I think that sequels are great, especially if they follow your rules! However, I feel that a lot of times the girl gets the guy in the first book and after that I become bored with their relationship, so I like it when the author moves onto two different characters.