Playing Byzantium in late 1200s. My vassal dukes are starting to gain control of counties in other duchies. What are the most effective ways to deal with this? Revoking their duke title and giving it to a de jure count doesn't do much in the long-term, since the former duke tends to control more counties/have more allies and can simply rebel and retake the title. My Empress is going to die soon, and at this rate the succession crisis will be huge if said dukes don't like the heir.
Edit: Just fyi I have Medium Crown Authority
Edit: Just fyi I have Medium Crown Authority