to prevent duplicate email address registrations, you will need to create a recordset on the page, set the filtering options to filter the email column on the Emails_Address Form POST.
then double click the server validation server behavior to add a new validation.
set the validation type to Number, and enter the following for the server variable:
where <recordsetName> is the name of the recordset you create.
set the minimum number to -1 and the maximum number to 0.
this will cause the form validation to fail is any records are found.