it's of no comfort, but apparently you're not alone: Duplex problem in Word 2010
this is something that MS must address, it's fundamental to MSOffice printing procedure..
- it seems that the Vista/Windows7 Office set-up overrides the printer's own native settings
one suspects that there ought to be a switch somewhere, allowing your own printer driver
to continue with the duplexing instruction..
seems as if the OS and MSOffice2010 are trying to do too much, here..