More likely, people won't forget how to spell their email address, or make a mistake in typing it that isn't immediately obvious; they might not remember which of their emails they used to sign up, though, even if they only have a couple. No one else can sign up with an email address that belongs to you.
I've seen mistype rates of close to 20% without the second field. Still, my boss won't allow me to put in the second field because he feels it would hurt the user experience.
