How to enable duplex printing on HP 7610 with Linux (Ubuntu 13.04)

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http://stackoverflow.com – I have equipped my HP 7610 printer with the duplex extension, but the configuration dialogs of Ubuntu and Kubuntu still have the duplex option greyed out. Even re-installing the printer does not help. It seems the printer doesn't tell the computer that it is capable of duplex printing now, still the same configuration is used. Is there a way of enabling duplex printing for this model? Would it help to add somethig to the PPD file? What would this extension look like? (HowTos)