XF 1.3 Duplicate email?

We've just banned an account from our forum due to the user having 2 accounts with the same email address.
I thought that Xenforo did not allow this?
Doh! Apologies, it was a long domain name and one had an "i" in the middle of it whereas the other was an "a". First look at it made it very hard to tell. Apologies for wasting your time.