def identify_two_list_duplicates(item1_list, item2_list):
    """Identify duplicate items between two lists.

    :param item1_list: The first input list to be checked.
    :type item1_list: list
    :param item2_list: The second input list to be checked.
    :type item2_list: list
    :returns:
        The set of non-unique/duplicated items between the two lists
        (but not duplicate items within each list).
    :rtype: set
    """
    item1_set = set(item1_list)
    item2_set = set(item2_list)
    item3_set = item1_set & item2_set
    return item3_set
