I appreciate the replies.
As for setting, it's a space opera, so I have a decent freedom to just make things up, but I want to stick at least somewhat close to real-life language for the sake of verisimilitude.
I guess I'll just go with "Duke John", "Duchess Anna", and so on.
That would certainly be made up.
In the UK at least, Dukes are not referred to as "Duke Firstname" It's the Duke of PlaceName. Frex: The Duke of Denver. The King and Peers and friends would call him "Denver", unless they were very close friends and in private where it *might* be "Gerald" (first name) or a nick name.
The only titles where a first name is used are the honorary titles given to the children of Dukes and the first born son, and any daughters, of an Earl. These are Lord or Lady Firstname.
So the Duke of Denver has two sons: His firstborn son uses the Duke's lesser title--Viscount St. George. His second and subsequent sons are Lord Firstname. His daughters are Lady Firstname.
An Earl's first born son is Lord Firstname (unless there is a lesser title to use, such as Viscount Fauntleroy), his subsequent sons are Mr. Familylastname. His daughters are all Lady Firstname.
Other than those few Lord and Lady Firstnames, they're all addressed by their title by their friends, peers and superiors in public or if they aren't close.