I found a solution by copying parts of my old HP930C ppd-file to the ppd-file of the HP7610.
First I added the following before the line "*OpenUI *InputSlot/Media Source: PickOne":
*OpenUI *Duplex/Double-Sided Printing: PickOne
*OrderDependency: 10 AnySetup *Duplex
*DefaultDuplex: DuplexNoTumble
*Duplex DuplexNoTumble/Long Edge (Standard): "<</Duplex true/Tumble false>>setpagedevice"
*Duplex DuplexTumble/Short Edge (Flip): "<</Duplex true/Tumble true>>setpagedevice"
*Duplex None/Off: "<</Duplex false/Tumble false>>setpagedevice"
*CloseUI: *Duplex
Then I added the followoing before the line "*DefaultFont: Courier":
*OpenGroup: InstallableOptions/Installable Options
*OpenUI *OptionDuplex/Duplexer Installed: Boolean
*OrderDependency: 10 AnySetup *OptionDuplex
*DefaultOptionDuplex: True
*OptionDuplex False/Not Installed: ""
*OptionDuplex True/Installed: ""
*CloseUI: *OptionDuplex
*CloseGroup: InstallableOptions
That enabled me to tell the CUPS system that the duplex option is installed and to select duplex printing in applications. Unfortunately, all output came out "short edge/flip". I had to add the following after the line "*RequiresPageRegion All: True" to make CUPS turn one side of the page so that the output now is perfect:
*cupsEvenDuplex: True
*cupsBackSide: "Rotated"
*cupsFlipDuplex: True
Additionally, I reordered the page size entries to have all relevant ISO-standardized sizes at the top of the list. The result of the whole process can be downloaded.