How to use email as username

Hello
I'm using cb 1.8.1 on joomla 2.5.7
The username is the lastname on cb and when there is 2 users with the same name the 2nd one cannot save his informations on the form. a message told him there is already a register member.
I wonder how it could be possible to change the username and to use the email adress (which is unique) as username instead of the nam?
Thank's for your answer

Hi this should been written in the description:
go to cb configuration -> general: The last item gives you the possibility for
username, username or email, only email, others

Thank's so simple;-) just in front of me

I'm confused.
My understanding of this field -- Login Type -- is that it controls what can be used for the login. This doesn't force the email address into the username field. You still need to specify a unique username at registration.

I have seen hacks in the past to handle this but the ability to actually use the email address as the username is something that CB should support.