In the print dialogue, under options, the duplex options are greyed out and not selectable. So from there the duplex option cannot be changed.

This is on openSUSE Tumbleweed. See downstream bug:

 https://bugzilla.opensuse.org/show_bug.cgi?id=1152385

The bug is not present in openSUSE Leap 15.1 with Qt 5.9.

I can change the default options through hp-toolbox and then I can print double sided. If I change the default to double sided and I print from okular, I get the following error message:

"There are conflicts in duplex settings. Do you want to fix them?"

If I select "no" printing works fine.

The printer is a HP Color Laserjet Pro M252 DW. It is connected through a network.