CODE EXAMPLE FOR PHP

php get duplicate keys in array without using inbuilt function

$arr = array(3,5,2,5,3,9);
foreach($arr as $key => $val){
  //remove the item from the array in order 
  //to prevent printing duplicates twice
  unset($arr[$key]); 
  //now if another copy of this key still exists in the array 
  //print it since it's a dup
  if (in_array($val,$arr)){
    echo $val . " ";
  }
}