Comment 21 for bug 286014

Apparently, some of the issues mentioned in this bug are slowly getting fixed.

At least it seems to be possible to control the duplexing (at least from okular), although it is awkward to do so.
The QT print dialogue has its own duplexing option which is controlled independently from the printer one and that seems to default to no-duplex. This should default to the printer config setting.