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    Java

    Map<Integer, Integer> check = new HashMap<>();
            AtomicInteger rs = new AtomicInteger(0);
            ar.forEach(it -> {
                if (check.containsKey(it)) {
                    rs.getAndIncrement();
                    check.remove(it);
                } else {
                    check.put(it, it);
                }
            });
            return rs.get();