// duplicates; [2] a Builder.DuplicateKey that records the first duplicate key we encountered
    // for possible later use in exceptions, perhaps straight away.
    Object hashTablePlus = createHashTable(alternatingKeysAndValues, n, tableSize, 0);
    Object hashTable;
    if (hashTablePlus instanceof Object[]) {
      Object[] hashTableAndSizeAndDuplicate = (Object[]) hashTablePlus;