E-Mail Address is already registered!

Im using OC 1.4.9.1
If i register a new account with a mail adress that is already registered, there is NO message like this:
E-Mail Address is already registered!
Instead it creates a "new" account but with the old ID. So if a user knows the mail adress of another cutomer, he could log on as him by just using his adress in the registration process.

UPDATE: alright, it seems that this message DOES appear, but only if the case is exactly the same. so it matters if its written upper or lowercase even though its the same address.