I have the same problem with a 2605DN
I haven't tested this but the Reg key is HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Printers\HP Color LaserJet 2605 PCL6 Class Driver\DsDriver which has a value printduplexsupported that needs to be set to 01. If you set it in the printer manager then save the reg key by doing right click export on DSdriver in the left panel it will give you a file you can double click to set the key as part of your start up or desktop.