Since upgrading to RAS v16.2, duplex printing option ( Print on Both sides) is not being listed anymore in print preferences.
Parallels Development Team is aware about this issue and working on resolution
1. At Parallels Client got 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.