Off the top of my head I would suggest the following:

If the duplex proeprties are contained within the printer itself (and not by document) then create 2 printers (name one "Printer 1-duplex on" and the other "printer 2-duplex off"; well I'm sure you can come up with something better then these names!) and set one to duplex and one to normal. Then the users can select by name what printer they want it to to.