1. Run hp-toolbox, click on the printer and go to "Printer Settings" tab
2. Check if option "duplexer" is "on" in "Installable Options" or "Miscellaneous" settings
3. Check if you have a Page Size of type "A4 AutoDuplex" under "General" settings and if selecting it will enable Duplex printing.
Few things to check
1. Run hp-toolbox, click on the printer and go to "Printer Settings" tab
2. Check if option "duplexer" is "on" in "Installable Options" or "Miscellaneous" settings
3. Check if you have a Page Size of type "A4 AutoDuplex" under "General" settings and if selecting it will enable Duplex printing.
Regards,
Suma