I try to enable duplex print by default by setting:

@prince-pdf {
      -prince-pdf-duplex: duplex-flip-long-edge;
}

which does not work. Duplex is not active when printing.
(But if I activate duplex manually, long/short edge is then correctly selected depending on the value I set.)

I'm using latest Adobe Reader.

In Acrobat, under File > Properties > Advanced > Print Dialog Presets, you should see DuplexMode: Duplex Flip Long Edge.

Yes, duplex is set to long-edge, but its not activated. Do I have to set another property to enable it?

I can't find anything else in the PDF specification that might be relevant. If you have an example of a PDF that makes Adobe Reader behave differently, we could have a look.

I just compiled some samples and tested them on my Mac (Adobe Reader) and here it works. So this seems to be an issue with Adobe Reader on Windows. I'll check on another Windows PC later this week. Thank you.