private static @Nullable Integer findDuplicate(int[] array) {
    Set<Integer> seenElements = new LinkedHashSet<>();

    for (int element : array) {
      if (!seenElements.add(element)) {
        return element;
      }
    }

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