Assume a list:

List<String> items = Arrays.asList("apple", "banana", "apple", "orange", "banana", "grape");

Using Collectors.groupingBy + filter

Set<String> duplicates = items.stream()
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
    .entrySet().stream()
    .filter(e -> e.getValue() > 1)
    .map(Map.Entry::getKey)
    .collect(Collectors.toSet());

System.out.println(duplicates);

Output:

[apple, banana]

Summary:

• Collectors.groupingBy(Function.identity(), Collectors.counting()) counts occurrences of each element.
• .filter(e -> e.getValue() > 1) selects only duplicates.
• .map(Map.Entry::getKey) extracts the duplicate values.
• Result is a Set of duplicate elements.