Hi
Welcome to the HP Support Community. I'd be happy to assist you with the duplex printing issue.
Let's try these steps -
Download and install HP Print and Scan Doctor, it is a Windows tool to fix printer issues.
If the issue still persists, try re-installing the printer driver -
1. In the search box, type and open "Uninstall a Program".
2. Look for HP Printer software.
3. Right-click to uninstall.
4. In the search box, type and open “Devices and printers”.
5. In the Devices and Printers, look for your HP printer. If you see it listed right-click on it and choose “Delete” or “Remove Device”.
6. Open the run command with the "Windows key + R" key combo.
7. Type printui.exe /s and click Ok. (Note: there is a space in between “printui.exe” and “/s”).
8. Click on the "Drivers" tab.
9. Look for HP Printer driver. If you see it Click on it and click “Remove” at the bottom.
10. Select Ok.
11. Select Apply and Ok on the Print Server Properties windows.
12. Close Devices and Printers Once the software and drivers are removed.
13. Restart the computer.
14. Download and install the software and drivers from here for Windows 10 (64-bit).
15. Click on the downloaded file and follow the on-screen instructions.
You may also refer to this document - HP Printers - How to Print on Both Sides of the Paper (Windows) (Duplexing)
Hope this helps! Keep me posted.
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Have a great day!