The manual duplex function is implemented in the foo2xqx driver which you are using, not in CUPS. Therefore I move this bug to foo2zjs which is the package which contains this driver. Presence of the hplip-gui, hplip-cups, and hpijs-ppds packages has no influence on this bug, they are not used in your workflow.
The manual duplex function is implemented in the foo2xqx driver which you are using, not in CUPS. Therefore I move this bug to foo2zjs which is the package which contains this driver. Presence of the hplip-gui, hplip-cups, and hpijs-ppds packages has no influence on this bug, they are not used in your workflow.