If you don't know exactly, how many times your string is encoded, you can use this function:

<?php

function _utf8_decode($string)
{
  $tmp = $string;
  $count = 0;
  while (mb_detect_encoding($tmp)=="UTF-8")
  {
    $tmp = utf8_decode($tmp);
    $count++;
  }
  
  for ($i = 0; $i < $count-1 ; $i++)
  {
    $string = utf8_decode($string);
    
  }
  return $string;
  
}

?>