If following the strictly the rules defined about critical hits in 3.x nothing special happens when rolling another 20 on critical confirmation roll.
DM/GM's I've played with usually choose to stop the game play briefly and describe a rather detailed and impressive feat of battle prowess your character has just preformed through that attack. It's a nice way of acknowledging that you've done something special.
It's worth noting that in 3.x if you reach epic level (20+) the DM/GM can chose to instate a new rule from the "epic Level handbook" where nat-20's no longer count as automatic hits because the AC of an epic monster could be gigantic (50+).
So what you can do instead is allow nat-20 rolls to add together to try and overcome the epic monsters AC (2 nat-20 and a regular roll would be = 40 + regular roll).
This rule also stats that nat-1's aren't automatic fumbles you continue rolling until you don't get a nat-20 or nat-1. Nat-1's count it as a -20 for these purposes. (nat-1 and regular roll = -20 + regular roll)