Just because I'm bored, I did some calculations based on

Lee's VMD charts, the predicted case capacity of 9mm and each powder manufacturers starting loads...

9mm case capacity I find listed as 0.862 cm³. It seems to me that most cartridges (that I've measured) run about 10% over minimum numbers, so I'll go with 0.862 cm³x1.1, .9482 cm³.

Lee shows a VMD for W231 as .0931, Bullseye as .1064 and Power Pistol as .08891.

Hodgdon shows the starting load of Win231 and 115gr SGD as 4.7, max 5.1

Alliant shows 6.0 PP starting, max 6.7 and Bullseye 4.2 start and 4.7 max.

Okey dokey then...

A starting load of 231, double charged, would be 9.4gr and occupy .87514 cm³, so 92% case capacity. Max load, doubled, would be 10.2gr and occupy .94962cm³, or exactly 100% capacity.

Power Pistol would be between 12.0 and 13.4gr if double charged, occupying 1.066 and 1.19cm³, or from 112% to 125% capacity.

Bullseye would be 8.4 to 9.4 of doubled, occupying between .893 and 1.000 cm³, or 94 to 105% capacity.

Basically, there's a good chance that a true double charge WOULDN'T over flow the case, except with Power Pistol. The main points, though, being that:

1) I personally doubt a "true" double charge is your most likely mistake, as a (rightfully) paranoid newb.

2) Even if you did it without an actual overflow, it should be painfully obvious.

3)Mike is absolutely right in that you need to be careful and it won't happen anyway. I would add that if you CAN'T or WON'T be that careful, you really should find another hobby.

I was just curious how the numbers came out.