David Nebenzahl wrote:
Nope. Not the problem. It's inherent in the lousy HP feeder design. I've
never seen a LaserJet that would print on the back side of just-printed
sheets even when brand new.

I've done it all the time on LaserJet II, III, 4m, and 5 printers. (I
often keep a tray full of scrap paper that's already printed on one
side.) It works best if the paper has chance to cool and lose its curl,
though. If it's hot out of the machine you may have problems. Sometimes
if the weather is try the paper will come out with a static charge that
makes it stick together and misfeed, too.