Duplexing is a function of the printer, not the document. If you could set
the printer ID and duplexing property in a doc, what would happen if you
opened the doc on a system that didn't have that printer available?
Probably there's a way in VBA to enumerate the available printers, determine
if any have duplex capability, and setting the properties accordingly - but
the coding would not be trivial, not something you could record with the
Macro recorder.