If any of you have an HP printer that can only do manual duplex on Windows, you'll find it quite annoying that the HPLIP drivers don't seem to have a feature that allows you to do the same manual duplex in the Linux side of things.
Fortunately, you can achieve the same results by using hp-print two times with slightly different print settings.
Print all the even pages first using a reverse portrait and reversed printing order.
Insert the pages without changing the orientation back into the paper tray, and print all of the odd pages in normal order and normal page orientation.
That's it. You should now have a fully duplex printed document.