hmm... sounds like you need to search the database for what is already there. to do that, you'd :
$connection = mysql_connect($host, $user, $pass);
mysql_select_db($mydatabase, $connection);
Now you need to make a SQL query. It sounds like it should be something like:
$email -- this is the email addy to check for
$query = "SELECT * FROM ".$db." WHERE email = '$email'";
$result = mysql_query( $query, $connection );
where $db is the DB that contains the stored addresses.
$inDatabaseAlready = mysql_num_rows( $result );
thus, if (!($inDatabaseAlready)) you should add the email address to the list.
hope this helps.