List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 2, 3);
Set<Integer> set = new HashSet<>();
List<Integer> duplicates = new ArrayList<>();

for (Integer element : list) {
    if (!set.add(element)) {
        duplicates.add(element);
    }
}

System.out.println("Дубликаты: " + duplicates);

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 2, 3);
Map<Integer, Integer> countMap = new HashMap<>();
List<Integer> duplicates = new ArrayList<>();

for (Integer element : list) {
    countMap.put(element, countMap.getOrDefault(element, 0) + 1);
}

for (Map.Entry<Integer, Integer> entry : countMap.entrySet()) {
    if (entry.getValue() > 1) {
        duplicates.add(entry.getKey());
    }
}

System.out.println("Дубликаты: " + duplicates);

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 2, 3);
List<Integer> duplicates = list.stream()
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
        .entrySet().stream()
        .filter(entry -> entry.getValue() > 1)
        .map(Map.Entry::getKey)
        .collect(Collectors.toList());

System.out.println("Дубликаты: " + duplicates);

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 2, 3);
List<Integer> duplicates = list.stream()
        .filter(i -> Collections.frequency(list, i) > 1)
        .distinct()
        .collect(Collectors.toList());

System.out.println("Дубликаты: " + duplicates);