There's a point at which your bankroll welcomes or shuns variance compared to your skill level in a game you play. For example, I can do well in $1000NL online games and have a bankroll that can withstand the variance of that game, so no need to run it twice. Sometimes, there are also unsustainable game options, games that don't always run.
Once I jump to a $5000NL game that I think I have an edge in, that edge is almost always going to be smaller (and almost always higher variance) and I also have to consider the very real opportunity costs of that variance, i.e. If I lose a flip in a $5000NL game, I might have to spend thousands of hands at $1000NL just to break even because that good $5000NL game isn't always there.
So there's a skill versus bankroll requirement threshold whereat the variance effect of running it twice is significant. If you are comfortable with your bankroll fluctuations and you're handily beating a limit, running it twice is going to hamper your ability to maximize your growth rate because you actually want to embrace variance at that point. It sounds odd, but simply put, consider if you're playing 25NL and you know you can beat 200NL well enough, you want to get to 100NL as quickly as possible, and that means welcoming higher variance.
On the other hand, if you have 20 buy ins for a game you beat for 2.5bb, then you probably want to run it twice and lower the variance of the game, especially if you take money from your bankroll for rent, food, and the like.