Solved: How do I Check for duplicate entries in mysql

Hello
I have a script that inserts items into mysql which works fine. However, what I need to do is check for duplicate entries and if found, issue alert and halt. I have coded some php but cannot seem to incorporate into my existing script.

I would be grateful if someone could help with this. Many thanks.

Exisitng code which works fine

<?php session_start(); ?>

    <?php

    $con = mysql_connect("localhost","root","");
    if(!$con) { die('Could not connect: ' . mysql_error()); }
    mysql_select_db("sample", $con);
    $dept = $_POST['dept'];
    $custref = $_POST['sub'];
		
		
    $query = "SELECT * FROM boxes WHERE department = '".$dept."' AND status = 1 AND custref = '".$custref."'";
    $result = mysql_query($query) or die(mysql_error());
    $row = mysql_fetch_array($result) or die(mysql_error());
    $r = $row['custref'];
		
    $str =  json_encode($r);
    echo trim($str, '"');

?>

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New code to be i was thinking of using

<?php session_start();  ?>
    <?php

    $con = mysql_connect("localhost","root","");
    if(!$con) { die('Could not connect: ' . mysql_error()); }
    mysql_select_db("sample", $con);
		
    $custref = $_POST['sub'];
		
    $result = mysql_query("SELECT * FROM boxes WHERE custref = '".$custref."'");
    $found = mysql_num_rows($result);
		
     if ($found > 0)
		{
			
			echo "true";
			
		} else {
			
			echo "false";
			
		}

     ?>

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jQuery code

$(function() {
				
$('#srcsubmit').click(function(e) {
e.preventDefault();
				 			 
if ($('#srcBox').val() == '') {

        notif({
        type: "error",
        msg: "<b>ERROR:<br /><br />You must enter a search term</b><p>Click anywhere to close</p>",
        height: 99,
        multiline: true,
        position: "middle,center",
        fade: true,
        timeout: 3000

        });
        return false;
        }
				 
				 
        $( "#submit" ).prop( "disabled", true );
        $( "#submit2" ).prop( "disabled", true );
        $( "#submit3" ).prop( "disabled", true );

        var value = $('#srcBox').val();
        var dept = '<?php echo $_GET['dept']; ?>';
				 
        var qString = 'sub=' + encodeURIComponent(value) + '&dept=' +encodeURIComponent(dept);

        $.post('sub_db_handler.php', qString, processResponse);
         });

   function processResponse(data) {
    
        $('#srcBoxRslt').val(data);

   };
		
});
		
	
</script>

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