According to the spec sheet http://h20195.www2.hp.com/v2/GetPDF.aspx/4aa6-3797eee.pdf it can duplex A5. Are you able to duplex any type of media? If not it may be that when installed the driver did not detect that the printer had duplex capability. 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. Then check under device settings tab and make sure that it shows that the duplex unit is installed. Change to installed if it shows as not installed.

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