Comment 39 for bug 905147

I also suffer from paper wasted because of KDE's print dialogs defaulting to single-sided printing.

(In reply to comment #26)
> I'm pretty sure that the right way to save the options is in cups, so that all
> applications, including non-qt applications, save these settings.

AFAICS, the situation actually is that the default in CUPS *is* duplex printing. This *is* used successfully by other programs, just not by Qt's printing dialog. Actually, there are two ways to configure duplex mode: one is via the cups properties, available in KDE's systemsettings and by pressing "properties" right next to the selected printer in the print dialog. There, duplex is selected. (This is the same thing as in comment #7 from Brice, only the other way round.)

However, the specialized GUI hidden behind the "options" button, in the settings tab, defaults to duplex OFF.

Looking at the code, the duplex mode GUI is set up from a QPrinter:
http://qt.gitorious.org/qt/qt/blobs/4.7/src/gui/dialogs/qprintdialog_unix.cpp#line434

I am not sure where the QPrinter comes from in general, and how to fix this properly. (Maybe introducing a "DefaultDuplex" QPrinter mode which lets the GUI default to the CUPS setting?)

It actually looks more like a Qt bug to me, too.