That would be in your printer settings not in ProSeries. 

here is another post with same issue  (although it is for Lacerte I think same fix would apply)

  https://proconnect.intuit.com/community/lacerte-tax-discussions/discussion/why-is-my-organizer-print...

Duplex printing option is a setting in your Windows Printer options. You need to turn off 2 sided printing in your printer settings, theres no option for this in ProSeries.