The Samsung ML2150 does that as does it's networked version (ML2151N). Those have duplexing STANDARD, not an add-on 'option' that costs you more. You should be able to find the 2150 for about $400 (possibly less if you look). It has more, better, features than any HP that tries to compete with it at the same price point. I was looking at HP when I wanted a networked laser printer, NONE came even close for the same kind of money. Pretty sad, since I've used HP printers before and they usually did fairly well.
BTW, the 2150 and 2151N both have duty cycles of 100,000 pages per month. That's more than most people will ever put through them in normal conditions. Medium offices could hit that, but it would mean they are printing 5,000 pages per day, per printer and not many places do that until you get to larger companies. even then, that's a hell of a lot of paper to be burning through.