Some Laserjet printers offer a feature known as "Alternative Letterhead Mode" which means that Letterhead paper can be loaded in the tray(s) the same way for Simplex and Duplex jobs; with the option disabled (or not available), 'sided' paper (like Letterhead, pre-punched, etc.) usually has to be loaded one way for Simplex jobs, but a different way for Duplex jobs.
Not sure if this applies to your printer.