It's often faster to use a foreache and array_keys than array_unique:

    <?php

    $max = 1000000;
    $arr = range(1,$max,3);
    $arr2 = range(1,$max,2);
    $arr = array_merge($arr,$arr2);

    $time = -microtime(true);
    $res1 = array_unique($arr);
    $time += microtime(true);
    echo "deduped to ".count($res1)." in ".$time;
    // deduped to 666667 in 32.300781965256

    $time = -microtime(true);
    $res2 = array();
    foreach($arr as $key=>$val) {    
        $res2[$val] = true;
    }
    $res2 = array_keys($res2);
    $time += microtime(true);
    echo "<br />deduped to ".count($res2)." in ".$time;
    // deduped to 666667 in 0.84372591972351

    ?>