The DUCHMAN(N) in Lancaster Co, PA, are descended from DUCHMANN Hans Adam (1728-1782) and his wife SCHLICHTER Christina (1728-1815), who emigrated from Alsace in 1766. Adam was the younger (and only surviving) brother of my ancestor DUCHMANN Johann Christman (1719-1802), whose descendants are all, or nearly all, known as TOOTHMAN in the USA. Adam and Christina lived at New Holland, Earl Twp, Lancaster Co.
I don't have a great deal on their descendants but I am not aware of any other DUCHMANN in PA at that time. The name is not especially common anywhere (except at a couple of villages in Alsace).