<?php

$arr = array(12, 46, 244, 0, 12, 83, 48, 98, 233, 83, 26, 91, 119, 148, 98);

function first_dupe($arr) {
    $res = array_unique($arr);

    foreach ($arr as $key => $val) {
        if ($res[$key] !== $val)
            return $val;
    }
}

echo first_dupe($arr);
?>

12