How to remove all unique elements from a collection?

Question

Java

How can I gracefully remove all unique data from a list?
For example, from this:

List<String> list = new ArrayList<>();
 list.add("A");
 list.add("B");
 list.add("C");
 list.add("A");
 list.add("B");
 list.add("X");

That is, all unique values ​​\u200b\u200bmust be deleted - C and X in this case.

Answer 1

list = list.stream()
            .collect(Collectors.groupingBy(
                Function.identity(),
                Collectors.counting()))
            .entrySet()
            .stream()
            .filter(e -> e.getValue() > 1)
            .map(Map.Entry::getKey)
            .collect(Collectors.toList());