I reload on a Dillon 550B (progressive with a manual indexed shellplate). A double charge occurs when the handle is pulled twice without indexing the shellplate. I've come close before when I first started but luckily caught my mistake before actually doubling. It seems that most of the problems are likely to occur when an issue arises during the loading process, i.e, primer slide bar doesn't pickup a primer, primer goes in sidewards, spent primer gets stuck under the shellplate instead of getting dropped in the cup, etc.. Somehow, something interrups the normal rhythmn/flow and forces you to pull a shell out of one of the stations. Trying to figure out where that casing goes after correcting the problem should seem to be easy enough, however this is normally where mistakes are made. I've learned that for me, its easier to just pull that one case, continue to finish loading the remaining cases on the plate until I have an empty shellplate, then start from sta. 1 all over. Along the same lines, if you continually pull cases from the plate to check and recheck OAL, crimp, etc. while loading, this tends to produce the same problem. I check powder charge before I start, then after every 100 complete cartridges (primer mag only holds 100 so I have to stop to refill anyway). Likewise, I spot check OAL/setback after every 100 also.
I really like my 550, it can produce a lot of ammo in a relatively short amount of time. Just have to be careful and pay attention.