You should not use deque.contains() method, it takes long time because it traverses through all array in every loop. I used map instead.

    Scanner in = new Scanner(System.in);
    Deque deque = new ArrayDeque<>();
    int n = in.nextInt();
    int m = in.nextInt();

    int max = 0;
    int curr = 0;
    Map<Integer, Integer> map = new HashMap<>();

    for (int i = 0; i < n; i++) {
        int num = in.nextInt();
        if (deque.size() == m) {
            int last = deque.pollLast();
            int val = map.get(last) - 1;
            map.put(last, val);
            if (val == 0) {
                --curr;
            }
        }
        if (!map.containsKey(num) || map.get(num) == 0) {
            ++curr;
        }
        deque.offerFirst(num);
        max = Math.max(max, curr);

        // add to map
        Integer val = map.get(num);
        if (val == null) val = new Integer(0);
        map.put(num, val+1);
    }

    System.out.println(max);