If they know there's a mistake. This is pretty much industry standard now to have to boxes and run a check that they match. Give me a few on this Houston. I'll see if I can't dig something up we can start with.
I can't believe there isn't two email address fields on the sign up form by default. Geeze... I'm gone for a few short years and things like this are allowed to happen. Two email address fields, or at least the option thereof, are essential.