I actually found a solution for it in excel before writing the following:

  asort($data);
  $duplicate = function ($val) use (&$data) {
    $count = 0;
    foreach ($data as $i => $entry)
      if ($val == $entry) $count++;
    return $count;
  };

  foreach ($data as $v)
    if (($count = $duplicate($v)) > 1) {
      printf("%d, Count %d\n", $v, $count);
      break;
    }

I assume it would be possible to use a sort+callback to return the value as well.