I had the same problem with my HP OfficeJet Pro 8620 and could solve it thanks to the replies above.
I'm attaching a modified PPD-file for the 8620 that enables Duplex printing (I took the original PPD and removed the constraints concerning Duplex).
I had the same problem with my HP OfficeJet Pro 8620 and could solve it thanks to the replies above.
I'm attaching a modified PPD-file for the 8620 that enables Duplex printing (I took the original PPD and removed the constraints concerning Duplex).