I found the simplest way to "unique" multidimensional arrays as follows:
<?php
$array = array(
'a' => array(1, 2),
'b' => array(1, 2),
'c' => array(2, 2),
'd' => array(2, 1),
'e' => array(1, 1),
);
$array = array_map('json_encode', $array);
$array = array_unique($array);
$array = array_map('json_decode', $array);
print_r($array);
?>
As you can see "b" will be removed without any errors or notices.