def identify_one_list_duplicates(item_list):
    """Identify duplicate items within a list.

    :param item_list: The input list to be checked.
    :type item_list: list
    :returns: The set of non-unique/duplicated items.
    :rtype: set
    """
    duplicate_items = set()
    item_set = set(item_list)
    for item1 in item_set:
        count = 0
        for item2 in item_list:
            if item1 == item2:
                count += 1
        if count > 1:
            duplicate_items.add(item1)
    return duplicate_items
