Binary package hint: okular
Okular's printing dialog does not use a printer's default duplex setting. My two printers are configured in CUPS to use "long edge" for two-side printing by default. In the GNOME printing dialog, this setting is always used as default, i.e. each time the GNOME printing dialog is opened, the duplex setting defaults to "long edge" (as it should). However, the KDE print dialog (tested with Okular) does *not* use this default but always takes "duplex off" as default. To print a document in duplex mode in a KDE application, it is always necessary to manually select the "long edge" mode in the KDE printer dialog, each time the dialog is opened. This annoying problem occurs at least under Lucid (KDE 4.4.x) and the latest Maverick beta (KDE 4.5.1).
The bug is *not* a duplicate of bug #286014. The printer dialog actually allows duplex printing (and duplex printing works correctly). The problem is that each time the printing dialog is invoked, it is again necessary to open the options, go to the settings tab and set the duplex option from "none" to "long edge". The printing dialog fails to get the default configured by CUPS (which is "long edge") and instead starts with duplex off.