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I too would assume Duckweiler and Dockweiler share the same origins. I descend from Christian Dockweiler who lived in Schleswig-Holstein. Keep in mind that surnames were in no way standardized until recently. Therefore, it is possible to see a surname spelled various ways - even in the same record. Also, variations can occur when interpreting the old handwriting (researchers in Schleswig-Holstein have interpreted the records to read Dunckweiler, Dockweiler, Wutiweiler etc). Also, in America, names often changed with time - as the family became more Americanized or as different clerks tried to interpret a foreign name.
As a bit of history, I would assume your ancestors came to America in the early to mid 1700's. Many (but certainly not all) Germans came through Philadelphia. Some stayed in PA and others migrated into MD, VA & the Carolinas. A large proportion of Germans who came to America at that time came from Pfalz, Baden-Wurttemberg and Alsace - areas that suffered greatly from wars, crop failures, religious strife etc. Not all emigration from those areas was to America. Schleswig-Holstein (then under Danish control) was a popular destination because of available land. My Christian Dockweiler migrated there from Eggenstein bei Karlsruhe. Other destinations were eastern Europe and the Ukraine.
Sorry I can't give any details on your family but hope the above is of interest.