gistfile1.php

<?php

$list = array(
  array(
    'foo' => 'caca'
  ),
  array(
    'foo' => 'prout'
  ),
  array(
    'foo' => 'caca'
  ),
  array(
    'foo' => 'prout'
  )
);

$newTab = array();
foreach ($list as $item) isset($newTab[$item['foo']]) OR $newTab[$item['foo']] = $item['foo'];

print_r($newTab);

// => Array ( [caca] => caca [prout] => prout )