I don't know if any filters are out there to do the duplex printing, but all
it takes is four lines of post script code, so all you need is a little
knowlege of perl to filter the job. 

You may want to look into the ifhp filters (I think they're included with
LPRNG http://www.lprng.com/) 

I think this should work for most duplex printers that speak postscript: 

%%BeginSetup
2 dict dup /Duplex true put dup /Tumble false put
setpagedevice
%%EndSetup

These commands should go after "%%EndComments" in your ps file. After you
edit the ps file, send it the the printer. More information is here:
http://www-sphys.unil.ch/linux/netprintfilter.html

The trick is writing a filter to insert the commands at the correct place.
Shouldn't be too difficult, but I haven't had the ambition. 

--
Andy Zbikowski, Sys Admin   | (WEB) http://www.ltiflex.com
LTI Flexible Products, Inc. | (PH)  763-428-9119 (EX) 132
21801 Industrial Blvd       | (FX)  763-428-9126
Rogers, MN  55374           | (PCS) 612-306-6055