Form multiselect item and url encoding with GET method
I am not sure if this is gonna work, but I think it might help you a little bit.

$query = explode('&amp;', $_SERVER['QUERY_STRING']);
$combined = '';

foreach($query as $key =&gt; $value) {
  if ($key == 'option') {
    $combined .= $value;
  }
}

drupal_goto($_SERVER['REQUEST_URI'] . '&amp;option=' . $combined);