The 'automatic discovery' feature used by some (Windows) drivers on installation (or update) doesn't always work; ensure that the driver knows that a duplex unit is present on the printer (assuming that one is present):
- For the printer instance in question, bring up the Printer properties dialogue.
- Select the Device settings tab.
- Ensure that the duplex unit is set to Installed rather than Not installed.
You'll probably need to be logged in with administrator credentials in order to change the device settings.