Thank you for the fast reply!
From the reference bug report http://bugzilla.gnome.org/show_bug.cgi?id=520598
it seems that you wrote the program because there was no option in evince for manual duplex. In kde-land this option was there in the print dialog gui of okular (the kde equivalent of evince) and it was working perfectly with your clp-300 driver. I just wanted to know what is the cause of the regression.
Until it is fixed your program seems to be a good alternative, thank you for making it. But in the long run I feel it an unnecessary extra step if duplex printing was already working with kde before.