I'm losing count, but I think that the replica Robertson post mill at
Colonial Williamsburg has itself been replicated in 3 other locations.
(Well when you have a good set of blueprints, it's worth reusing them!)
The latest version has recently been completed at the Mid-America
Windmill Museum, Kendalville - a location more generally associated with
American wind-engines. The museum is holding its
4th Annual Windmill Festival from 27-29th June, with lots of
activities including crafters, music, and Civil War reenactments.
The mill was constructed with much help from the Timber Framers Guild, and
there are more details on the
museum site
Latest Robertson windmill completed at Kendallville, USA