code: protected final void assertDoesNotHaveDuplicates() {
      isNotNull();
      Collection<?> duplicates = duplicatesFrom(actualAsList());
      if (duplicates.isEmpty()) {
        return;
      }
      failIfCustomMessageIsSet();
      throw failure(format("<%s> contains duplicate(s):<%s>", actual, duplicates));
    }
docs: Verifies that the actual group of objects does not have duplicates.
  
  @throws AssertionError if the actual group of objects is {@code null}.
  @throws AssertionError if the actual group of objects has duplicates.