How to find duplicate elements in an array using single loop. Interview question PHP

Mostly ask question in php interview.

<?php
$arr = [12,32,32,12,33,3,33,33];
foreach(array_count_values($arr) as $val => $freq) {
    if($freq > 1)
 {
  echo "Duplicate value:". $val . "<br>";
 } 

}