MySQL error 1062: Duplicate Entry

Hey everyone, I was working on an online registration section of a cilents website. Everthing was working fine till I got this error message:

MySQL error 1062: Duplicate Entry ‘4428’ for key 1

This is really messing my project up. What does this mean? What should I be looking for to fix this. Also here is the the code where the problem is coming from:

$db = mysql_select_db($db_order_name);

    $query = "INSERT INTO orders
            SET studentID = '$studentID',
            firstname = '$_POST[firstname]',
            lastname = '$_POST[lastname]',
            schoolID = '$_POST[schoolID]',
            parentname = '$_POST[parentname]',
            address1 = '$_POST[address1]',
            address2 = '$_POST[address2]',
            city = '$_POST[city]',
            state = '$_POST[state]',
            zip = '$_POST[zip]',
            email = '$_POST[email]',
            homephone = '$_POST[homephone]',
            workphone = '$_POST[workphone]',
            cellphone = '$_POST[cellphone]',
            emergencyphone = '$_POST[emergencyphone]',
            emergencyname = '$_POST[emergencyname]',
            grade = '$_POST[grade]',
            teachername = '$_POST[teachername]',
            asp = '$_POST[asp]',
            notes = '$_POST[notes]',
            newstudent = '$_POST[newstudent]',
            addedby = 'customer',
            ordernumber = '$orderNumber',
            periods = '$_POST[period]',
            periodname = '$periodname'";

    $result = mysql_query($query); <==== It think it is happening here

If you need more just let me know I’m a noob to php, mySQL and kirupa’s forum.