create a recordset to lookup the entered email address.
then edit the trigger code for the insert record behavior.
if the trigger is set to any form submit, it will look like:
if ($_SERVER["REQUEST_METHOD"] == "POST") // Trigger
you need to add a second condition to check if the recordset is empty:
if ($_SERVER["REQUEST_METHOD"] == "POST" && $totalRows_RecordsetName == 0) // Trigger
where RecordsetName is the name of the recordset.