CUPS EvenDuplex
Another remind-myself blog-post.
If you've got a printer like mine, which accepts PCL6, and expects there to always be an even number of pages
when doing duplex printing, CUPS has a very easy solution for you. Open-source software usually always has.
*cupsEvenDuplex: True
Add that to your PPD file, and voilĂ , enjoy duplex working also when submitting three pages for printing!