_duplicate_arg() not implemented right

I get a warning message:

Undefined variable: index in E:\esg\sites\all\modules\spam\filters\duplicate\duplicate.module:70

This is due to bad implementation of _duplicate_arg:

function _duplicate_arg($index = null) {
  static $arguments, $q;

  if (empty($arguments) || $q != $_GET['q']) {
    $arguments = explode('/', $_GET['q']);
    $q = $_GET['q'];
  }

  if (!isset($index)) {
    return $arguments; 
  } 

  if (isset($arguments[$index])) {
    return $arguments[$index];
  }
}

the middle if and the parameter $index is missing.

I'll attach my patch.