When using Okular 1.3.3 on OpenSuse 15.0 with the Postscript driver, printing takes unbelievable long (20+ min/page sometimes, sometimes never stops spooling) but mostly works. If printer produces any output, duplex-ing with Okular works fine. When using Okular 1.3.3 on OpenSuse 15.0 with the PCL based driver, printing itself goes pretty fast, but DUPLEX settings are ignored when using Okular. Same printer driver is doing duplex-jobs fine when e.g. using xreader instead of Okular. Or any other software like editors, LibreOffice, Firefox etc., so I'm convinced that this must be an Okular bug.
Issue can be closed. Happens only with (old) driver "HP LaserJet 2200 Series, hpcups 3.15.2", seems to be kept when upgrading from older OpenSuse versions. OpenSuse 15.0 now has "HP LaserJet 2200 pcl3, hpcups 3.17.9" available, WORKS FINE with this driver, also from Okular :-)
Thanks for the update; closing.