This may be a bug, but I don't know, so don't bite my head off.
On stable islands the dukes of cities have that title appear twice in messages, contact lists, etc. So there'll be "Sir Maurus Barek, Duke of Nocaneb, Duke of Nocaneb, Ambassador of Arcaea". Is this a bug or something to do with the new estate system, where the lord of a city and the duke of a duchy are two separate titles?