Obtener valor duplicado de la matriz PHP
$arr = array(1, 4, 6, 1, 8, 9, 4, 6);
$unique = array_unique($arr);
$duplicates = array_diff_assoc($arr, $unique);
print_r($duplicates);
Array ( [3] => 1 [6] => 4 [7] => 6 )
Easy Elephant