A customer that had been printing duplex documents to a HP LaserJet 8150 had to send the printer off for repairs.  When they got it back and reconnected it to the network, they were unable to print duplex.  Printing test pages from the printer’s console came out duplexed and the settings on the display showed that duplexing was enabled. 

When I went to look at the printer properties on the printer server, I found a setting under the Device Settings tab for Duplex Unit.  It was set to Not Installed.  As soon as I changed it to Installed, users were able to print on both sides of the page.  I’m not sure what caused the printer to lose this functionality while it was being repaired, but this was the solution. [more]