[mille562]

Some may suggest to convert the email address into it's base form and store it in an extra field. So [e-mail] would become [e-mail]. Use the extra field to block any new signups that match an existing base address. But this has a high chance of blocking valid email addresses. Different email providers allow different aliases: plus, hyphen, dot, equals. That, combined without knowing which each email service a domain is using, makes this method very difficult to get right.