Have you checked the driver to ensure that it still thinks that the printer has duplex ability. On your PC:
go to control panel>devices and printers and right click on the icon for your printer and select printer properties from the drop down menu. Click on print preferences
If you see the "print on both sides" option you can turn it on and all documents will duplex. If you do not see or cannot select that option then click cancel and go to device settings and make sure that it shows the duplex unit as installed. Once you do this you should be able to set duplex as the default preference as above.
Once you have done this, right click on the icon for your printer again and select "set as default" even if the printer is already your default. When you do this, you will be warned that doing this will disable Microsoft Print Mangement, say yes, this is what is causing you to lose your duplex options.
If you find the information provided useful or solves your problems, help other users find the solution easier by giving kudos and marking my post as an accepted solution.