check for duplicate value in input

I have code that checks mysql for duplicate entries in a database and issues error through ajax. That works fine. My problem is that I have no way for checking duplicate entries in the form when the form is sent to php for processing. The inputs are created by a jquery change on a slider so there could many inputs created by the user from which they create unique codes. What I am trying to do is check that if a user enters the same number in the form then issue an error. Am I better doing this before it reaches php using jquery?  I hope someone can help with this. Thanks

$duplicates = array();

foreach ($boxnumber as $val)
{
if ( $val != "" )
 {
  mysql_select_db($database_logistor, $logistor);
  $sql = "SELECT custref FROM boxes WHERE custref='$val' and status = 'In'";
  $qry = mysql_query($sql) or die(mysql_error());

  if(mysql_num_rows($qry) < 1)
  {
  $duplicates[] = '[ '.$val.' ]';
  $flag=1;
  }
  
 }
}

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