In the past the Lord of the City and the Duke of the Duchy were the same. In the new system, those two positions are separate. Lord of City does NOT have to mean Duke of Duchy. The Lord of a City (or any region, Cities don't have a special status any more in this context) can be a different Noble then the Duke of the Duchy. Should the Duke decide he appoints himself as Lord of that City (or any other region), then he's also the Lord yes, but you can be a Duke without directly controlling a region. You can't own more then one region.
