Suppose the difference is Alexander raised cities and named them Alexandria. However historically there weren't Duchies that was called the Duchy of Robertia, then Duchy of Michaelia, then Duchy of Johnia. You wouldn't see the Duchy of Normandy, or Duchy of Lancashire suddenly be renamed to something else because of the holder having this name or that. If anything, cities and such should have an option to rename after a fallen hero or the like.
Well considering this isn't a !@#$ing medieval simulator, plus such lofty examples as Duchy of Atreides, Duchy of Harkonnen, etc. from Frank Herbert's Dune provide fictional examples of duchies named after people.
