Okular is a PDF viewer for the KDE platform. When printing a PDF from Okular, duplex printing (printing on both sides of a sheet) can be enabled in the Options section for a chosen printer. For a printer that has duplex support (for example, HP Color LaserJet 4700), Okular shows this duplex printing section as disabled. So, I am not able to print duplex to this printer from Okular. Other duplex printers (for example, HP Color LaserJet Enterprise CP4520) show up with this section enabled in Okular.
I could not figure out why this problem happens. I am guessing it has something to do with the printer driver and its communication with the printer backend used by Okular.
In any case, I was able to print by opening the PDF using Evince, which is known as Document Viewer in Ubuntu. The print dialog in this had the duplex option for the same printer and I was able to print it.
Tried with: Evince 3.4.0, Okular 0.14.3 and Ubuntu 12.04 LTS