Read the information here...
http://www.math.tamu.edu/~boas/courses/math696/duplex.htm
Look at a spool file generated on your system and see exactly your duplex option adds to the file. Save small files created with and without duplex options.
Experiment a bit to see if you can manually edit a file as proposed in the article above and make it duplex, and then see what is lacking in the output file as it stands.
BTW, is there a PJL wrapper put around the job with an ENTER LANGUAGE = POSTSCRIPT command immediately before the postscript data?