From what I've read here in the forum, not all printers can print "duplex" and FileMaker does not always print successfully in duplex mode with those printers that can duplex print.
You may have to do it "old school" where you make print run number 1 to print one side, reload the paper with it flipped over and do print run number 2 to print the other side. Obviously, care must be taken to make sure the front and back sides of the document come from the same record. A misfeed of just one page can waste a lot of paper...
with windows xp I cannot use the "duplex option", but with a mac OS X yes.