Definition of run it twice
"Run it twice" means "roll it twice". In more precise poker terms, a run it twice occurs when 2 or more players are all in and they decide to "run it twice" i.e. reveal 2 boards. If, for example, two players are all in on the flop and the equity of the two hands is about 50-50, the players may conclude to run it twice and therefore take out a turn and a river for 50% of the pot and then a second turn and a second river for the other 50% of the pot.
Should you run it twice?
If the poker room doesn't take more commission for the run it twice, that's up to you. The purpose of run it twice is to reduce the variance. If you are all in preflop in a 50-50, there is no reason to refuse a run it twice. By doing the run it twice, it's like playing the hand twice. And the best cure for run bad is volume. If you are a long-term winning player, the more you play, the more you will win.