printer not accepting two-sided printing

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http://askubuntu.com – On a network printer I can only print on one side, not on both, even though the printer is perfectly able to do so. For instance, when on evince I select two-sided (either long or short edge), a red message appears saying that "some of the settings in the dialog conflict" and indeed it only prints on one side, whatever the setting I specify. What should I do? (HowTos)