Prevent multiple INSERTS due to click event

[BlueGhost]

Hello,
I have a div contained in an object that registers for a click event, and once clicked, uses ajax to communicate with the server and update a user rating for an item.

When updating the rating, it checks if the item already exists in the database, if not, I use an INSERT, otherwise I use an UPDATE. My issue is that if the user clicks very quickly, two INSERTS are run and i end up with duplicates in my database. If the ajax call has a chance to return between clicks then it works as expected with an UPDATE being run on all subsequent clicks. Is there a standard means of preventing this or disabling the onclick and re-enabling it on the ajax call back?

Here is my abbreviated code:

$sql="UPDATE database SET rating=" . $_POST['rating'] . ", status=" . $_POST['status'] . " WHERE user=" . $_POST['user'] . " AND isbn=" . $_POST['isbn'];

    $result = mysql_query($sql);       
    if (mysql_affected_rows()==0) {
        $sql="INSERT INTO database (user, isbn, rating, status) VALUES (" . $_POST['user'] . ", " . $_POST['isbn'] . ", " . $_POST['rating'] . ", " . $_POST['status'] . ")";
            if (!mysql_query($sql, $connection))
        {
            echo("------Book failed to add------");
        }
        else
        {
            echo("------Book successfully added------");
        }
    }
    else
    {
        echo("------Book successfully updated------");
    } 

//add event listener
star1.addEventListener('click', function(e){changeYouStars(e, mySelf);}, false);

[NogDog]

One way to approach it would be to set a unique index on that DB table across whatever columns would define a uniquely identifiable row (maybe `user` and `isbn`?). Then you could always do an insert, but add an ON DUPLICATE KEY UPDATE clause to the query.