I double charged a couple last night. No big deal, I was not doing production, but special test loads. But let me explain - wanted to leave the press setup as is, so I was pulling cases from station 2 (powder drop) and hand weighing a different powder into each case. I needed to bell the cases, so I would get the setup charge and dump it into the hopper, then load the test charge. Unfortunately, I came across a couple of cases with loose primer pockets and pulling those, removing the primers for reuse, and in doing so ended up with a couple of double charges which completely filled the cases without spilling, but were getting dumped into the hopper anyway.
I have loaded a lot of rounds without double charges, and many of those were with Bullseye in large cases. It's doubly important to pay attention to loads with a low case charge.
But, I have taken to finding loads where a double charge is obvious, I have good lighting at the press, and I visually inspect every round. A good rythym and following the same sequence of operation every time doesn't produce double charges.
I'd bet that double charged rounds are simply the result of inattention to detail and/or taking short cuts, and that putting those same individuals on a single stage press will eventually produce a squib load they fail to notice at the range, with unfortunate results.