I have the HP OfficeJet Pro 8500A Plus and it always printer duplex correctly for me. So can you please try the following: Manually change the printer model/driver of your print queue to the HP OfficeJet Pro 8500A Plus.
To do so, open system-config-printer ("Printers" icon in System Settings), right-click the icon representing your HP OJ Pro 8600, choose "Properties" in the pop-up menu, then click on the "Change" button of "Make and Model" (on the "Settings" screen). Choose manufacturer "HP" and model "HP OfficeJet Pro 8500A Plus". Then choose the "hpcups" driver (is probably already pre-selected). Click "Forward" or "OK" to close the dialog.
Now try to print. Does duplex work correctly with all applications now?
Other possibility worth trying is to choose the "hpijs" driver, for both model selections "HP OfficeJet Pro 8500A Plus" and "HP OfficeJet Pro 8600". Please tell whether this helps.
I have the HP OfficeJet Pro 8500A Plus and it always printer duplex correctly for me. So can you please try the following: Manually change the printer model/driver of your print queue to the HP OfficeJet Pro 8500A Plus.
To do so, open system- config- printer ("Printers" icon in System Settings), right-click the icon representing your HP OJ Pro 8600, choose "Properties" in the pop-up menu, then click on the "Change" button of "Make and Model" (on the "Settings" screen). Choose manufacturer "HP" and model "HP OfficeJet Pro 8500A Plus". Then choose the "hpcups" driver (is probably already pre-selected). Click "Forward" or "OK" to close the dialog.
Now try to print. Does duplex work correctly with all applications now?
Other possibility worth trying is to choose the "hpijs" driver, for both model selections "HP OfficeJet Pro 8500A Plus" and "HP OfficeJet Pro 8600". Please tell whether this helps.