<?php
$array   =  array(1900,1900,2000);
$array2  =  array();

foreach ($array as $val) {
    if (!in_array($val, $array2)) {
        $array2[] = $val;
    }
}

print_r($array2);

Array
(
    [0] => 1900
    [1] => 2000
)