When the user submits the form - first do a call to the database
PHP Code:
$sql=select * from TABLE_NAME where email="'.$_POST[email].'";
$Result=mysql_query($sql, $link_id);
Then see if any records are returned (use mysql_num_rows($Result) )- if none then you know the email is unique and you can go ahead and accept it. If a record is returned from the query, then you know that the email address has already been used and display the appropriate error message to the user