How do I enable 2-sided printing with a Xerox DocuPrint laser printer?

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http://superuser.com – After installing the drivers for my DocuPrint C2255 printer, it just will not allow me to select double-sided printing. The PCL driver does not even display "2 sided" as an option in the drop-down The PostScript driver displays the option, but when selected gives this slightly confusing error... It seems to think that the duplex unit is not installed. What do I do? (HowTos)