Since upgrading to RAS v16.2, duplex printing option ( Print on Both sides) is not being listed anymore in print preferences.
Upgrade to the most recent version of Parallels Remote Application Server
1. At Parallels Client go to Connection Properties > Printing tab > Options > select Use client preferences for all printers
2. Re-login to Farm and launch the application. A local printer preferences dialog will open when you click Print in an application. The dialog will contain a full set of options for a particular printer that you have installed on your local computer.