Well, see, that's where things get muddled. If you have a Vintage set and a Millennial set and a MOTUC set, you can have three Castles Grayskull. I think in these cases, the proper name is actually considered an adjective modifying the title / building / whatever. And if you have, say, Tatooine redneck Luke standing next to Jedi Luke, you'd have two Lukes Skywalker.
That's how I learned it, anyway. Then again, my memory not being what it was, I may be misremembering.