}
    }

    private @Nullable Object[] lastEntryForEachKey(
        @Nullable Object[] localAlternatingKeysAndValues, int size) {
      Set<Object> seenKeys = new HashSet<>();
      BitSet dups = new BitSet(); // slots that are overridden by a later duplicate key
      for (int i = size - 1; i >= 0; i--) {
        Object key = requireNonNull(localAlternatingKeysAndValues[2 * i]);
        if (!seenKeys.add(key)) {

     * {@code null} is returned.
     */
    @CheckForNull
    private static <K, V> Entry<K, V>[] lastEntryForEachKey(Entry<K, V>[] entries, int size) {
      Set<K> seen = new HashSet<>();
      BitSet dups = new BitSet(); // slots that are overridden by a later duplicate key
      for (int i = size - 1; i >= 0; i--) {
        if (!seen.add(entries[i].getKey())) {
          dups.set(i);
        }
      }