Run it once or twice

[dride]

I did a quick search of this forum and didn't find anything relating to this. I play in a home game where when anyone is all in each player is asked whether they want to run it once or twice after the flop. If both players agree on running it twice then it is ran twice, if one player says run it twice and the other says run it once, then it is only run once.

In what situations is running it twice profitable or when once is profitable. As I understand it running it twice reduces variance so say you call someone with just a flush draw do you want to run it once or do you want to run it twice. Any help would be appreciated.