Actually with the Duron's, overclocking is not hoping you get a good one.
ANY normal Duron (90%+ of the ones made), will run at 900MHz. Doesn't matter if it's a 600 or an 800, most will run at 900MHz. A Good one will run at 1Ghz. A bad Duron will still run at a minimum of 750Mhz. Any Duron that will not run at 750MHz is defective, plain and simple, and most likely has problems with it's internal cache. The chances of this are very rare, probably around 1 in 500. Personally, with the 12 Duron 600 systems I've built, I've never seen a 600 NOT run 935MHz 100% stable, but these were all with Abit motherboards.