so you have a form that allows people to enter an email address to sign up for the news letter?
You would need to create a recordset on the page to lookup the email address that is entered in the form, then use the server validations portion of CSS Form builder to validatie that the recordset is empty (email has not already been added).
use number validation.
For the server variable enter "$totalRows_<recordsetName>" where <recordsetName> is the name of the recordset.
when setting up number validation, set the minimum number to -1, and the max to 0