I'd like to reemphasize that if you donate to multiple effective charities you are doing awesome stuff. Switching from average charities to a diversified portfolio of effective charities can make you hugely more effective -- it's like turning yourself into 10 people. Switching from a diversified portfolio of effective charities to the single most effective charity might make you maybe a few percentage points more effective*. That's not nearly as important as doing whatever makes your brain enthusiastic about effective altruism. The point I'm made in the parent comment is not of practical concern.

*I am making up these numbers -- don't quote me on this.

Not generally. It's usually just there to counteract overconfidence bias. You want something that will never fail instead of something that will fail 1% of the time, because something that you think will never fail will only fail about 1% of the time, and something that you think will fail 1% of the time will fail around 10% of the time. It's much more than the apparent 1% advantage.

If you donate all your money to Deworm the World because you want a lot of good to still get done if SCI turns out to be a fraud, you're not being selfish. If you donate half you money because you personally want to be doing the good even if one of them is a fraud, then you're selfish.

I alluded to this with the sentence:

That makes sense if you care about how many lives you save, but not if you care about how many people die.