I have an old HP Color LaserJet 2605dn in my office. When I first set it up in Windows 7, I used the HP Color LaserJet 2605/2605dn/2605dtn PS driver from Microsoft. It worked quite well until I tried to print something in duplex. I spent a lot of time trying to find the duplex option in the printer properties dialog and could not find it.
A post in the HP business forum pointed me to the right direction. To enable duplex printing of Color LaserJet 2605dn, you need to use the HP Universal Print Driver. Just go to HP’s support page at http://www.hp.com/#Support and enter the printer model to download the driver for installation.