Hello folks
I know it may seem obvious to many, but Okular is one of the few apps that does not print on both sides on both my PCs.
I have this option set up in HPLips
Every time I try, in advanced settings, to put "long edge" it reverses to "Off" (in the printing dialog box)
And this duplex setting, what is it in fact ? Any relation ?
[Edit 06-11-2015] after some updates and upgrades, the pb disappeared. It now works for me.

This thread is pretty old, but I'm having the same trouble using an HP printer and hplip over my network. My workaround is to use a GTK app to print using the same printer driver. E.g. I open pdfs in Evince (Document Viewer) and print them from there since duplex printing is grayed out in the print dialog for KDE apps like Okular.

I have no problem printing duplex from my networked Samsung printer using its driver. So it seems to be an HPLIP thing.

UPDATE: Enter the CUPS interface via your browser. Go to Administration. Click "Manage Printers". Select your printer. Under "Administration" click "Set Default Options". Enable duplex printing.

This worked for me anyway. Source: https://forums.gentoo.org/viewtopic-t-1 ... 81ec879a47