Yes, it is a known bug. It's a relic of the way the messaging system works, and some of the code that is shared between testing and stable islands. In order to get the separation of Duke and Lord-of-a-city titles to work properly on Testing islands, some common code had to be changed. This resulted in the "Duke of Kepler, Duke of Kepler" problem on stable islands. It will get fixed when the new estate system is ported to stable islands.
