Comment 25 for bug 428588

Michal, pleas check at first whether the printer is configured for having a duplex unit, start the printer tool with System -> Administration -> Printing, right-click the printer, choose "Properties". Choose "Installed Options" on the left and make sure that the box at "Duplex Unit" is marked. Now you can set the default choice for double-sided printing under "Printer Options". Set the default page size to the "Duplex" variant of your default page size if you choose to print duplex by default. From now on applications should offer duplex printing in their dialogs.