code: private static @Nullable Integer findDuplicate(int[] array) {
      Set<Integer> seenElements = new LinkedHashSet<>();
  
      for (int element : array) {
        if (!seenElements.add(element)) {
          return element;
        }
      }
  
      return null;
    }
docs: Returns the first duplicate element inside an array, null if there are no duplicates.