Preco rekurzia?
Kód:
<?php
function uniq_name($name) {
  while (file_exists($name)) {
    $exploded = explode(".", $name);
    $exploded[count($exploded)-2].="2";
    $name = implode(".",$exploded);
  }
  return $name;
}

echo uniq_name("test.tst");

?>