If you have two Duplicates on the mat, a Rebuild, and 3 Duchies left on the board with no possible way of purchasing one or Rebuilding an Estate, you can deny the opponent the rest of them by Milling a Duchy for Duchy. Be forewarned that the opponent will now go Rebuild-BM and try to buy a Province or two, which ends up being worth a whole lot more than a Rebuilded Duchy > Province, so be quick with the pileout.