Here are steps I used to get duplex printing to work for the HP Color LaserJet 2605dn in either a Windows Vista or Windows 7 operating system:
Use the HP Color LaserJet 2605/2605dn/2605dtn PS driver that is already installed in the operating system
Type "Printers" in start search (just press Windows key and start typing)
Right-click the HP 2605 printer (whatever you named it) and select Properties
Click the Device Settings tab.
Scroll down to Installable Options section and change the setting for Duplex Unit (for 2-Sided Printing) from
Not Installed to Installed.
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