// Reject an order id we have already processed.

static List<Integer> seen = new ArrayList<>();

static boolean isDuplicate(Integer orderId) {

    for (Integer id : seen) {

        if (id == orderId) return true;

    }

    seen.add(orderId);

    return false;

}

// isDuplicate(70);  isDuplicate(70)  -> true

// isDuplicate(700); isDuplicate(700) -> false  (!)