Array
(
    [0] => Array
        (
            [0] => 18
            [1] => 19
            [2] => 20
            [3] => 21
            [4] => 22
            [5] => 23
            [6] => 24
        )

    [1] => Array
        (
            [0] => 7
            [1] => 8
            [2] => 9
            [3] => 10
        )

    [2] => Array
        (
            [0] => 15
            [1] => 16
            [2] => 17
            [3] => 18
            [4] => 19
            [5] => 20
            [6] => 21
            [7] => 22
            [8] => 23
            [9] => 24
        )

    [3] => Array
        (
            [0] => 2
            [1] => 3
            [2] => 4
            [3] => 5
        )

)

$arrays = Array(
    Array(18),
    Array(7,9,10),
    Array(12,22,33,34),
    Array(7,22,4,5)
);

$repeats = "";
$all_items = array();
foreach($arrays as $var=>$val){
    $all_items = array_merge($all_items, $val);
}

foreach($arrays as $var=>$val){
    foreach($val as $subvar=>$subval){
        if(in_array($subval, $all_items) && array_count_values($all_items)[$subval] > 1){
            $repeats .= '['.$var.']['.$subvar.']<br/>';
        }
    }
}

echo $repeats;