When we do anything dangerous such as shooting guns, reloading ammunition or jumping out of airplanes, it is very helpful to have *several* overlapping things that, any one of which, will prevent or catch an otherwise fatal mistake. For example, the NRA has three safety rules for shooting (see http://www.nrahq.org/education/guide.asp), each one of which will keep you from shooting yourself or anyone around you. Indeed, to hurt someone, you have to fail at all three rules simultaneously.
The key attribute here is that the rules overlap. A fatal mistake can only happen if all three rules are violated at the same time. Safety on top of safety on top of safety.
In reloading, we also need overlapping rules. Squibs and double-loads are possibly the two most dangerous errors that can happen (well, other than setting off that tube full of primers and igniting the three pound jug of Clays). To prevent squibs and double-loads from ending up in your pile of ammunition, there are a combination of things that can be helpful.
First, use a progressive reloader that auto-indexes. The Dillon 650, to name just one such progressive, has auto-indexing. It advances the shells automatically beneath the reloading dies and, thereby, makes a double-charge difficult (but not impossible). In theory, a squib should also be impossible but anyone can run out of powder and, on some progressives, a short stroke of the lever will also advance an empty shell right on past the powder drop. (It could be argued that auto-indexing makes a squib more likely. I won’t disagree with that.)
Second, a powder-check die right after the powder drop is a great idea. It basically measures the height of the powder in the shell and triggers an alarm if it is either too high or too low. On the 650 I use, the check won’t catch small errors in the drop but I have verified that it will catch double, and zero, loads. (I load 3.8 grains of Clays in my 45 ACP wad loads, incidentally. The shell is roughly half full.)
Third, look in each shell after the powder check and verify that the level of powder is about right. That’s not very precise, of course, and it’s easy to get distracted — Darn it, the case feeder is jammed again — but if you work at making this a habit, it will become your third safety, one that is not subject to mechanical foul up.
Any *one* of those checks can go awry but the remaining two should catch (or prevent) a squib or a double load. Only when all three go wrong at the same time will the process fail. Only when auto-indexing doesn’t do right, and the powder check die fails to sound the alarm (or is ignored!), and you don’t visually see (or wake up when!) the powder level is way off, only then will your process build a bad round.
Finally, let me add that I’ve personally witnessed one of the best shooters in the world blowing a hole in the firing point table when he failed to follow one of the NRA’s three safety rules. His round went harmlessly into the dirt away from everyone so the remaining two safety rules ensured everyone’s safety, but this does illustrate the point that if the rules overlap, it’s far less likely that someone will get hurt.
Repeating repetitious redundancy is good for you.