At the moment, according to the documentation, it is not possible to do duplex-printing on windows.
qprinter.setDuplex() // qprinter.duplex() only works on x11.

it would be really nice to have this feature also on windows ,)

its actually mandatory for me in order to use qt for my planned app, since i cant switch
easily to another os, due to printersupport issues. (plastic-card printing solution)

some feedback if thats going to happen at all, and if so, when it might happen - would be nice