That looks like an excellent casus belli to me. It's also historically accurate: see the numerous British Kings who claimed the title of "King of France", or the title of "Duke of Normandy" that the british crowns hold onto to this day.
yes.. but the thing is. those were existent titles. and they do have a blood link to it.
it's utterly stupid to have a kingdom of france.. and then someone from the other side of the world create a duchy of france.
i would even suggest ban all future ducal titles that matches existent region names. (ie.. existent ones keep... but no to new ones)
