Sort Array by Element Frequency doesn't work

Quest just does not work. https://py.checkio.org/mission/sort-array-by-element-frequency/solve/

My code: (it works properly) def frequency_sort(items: list) -> list: res = []

def ordered_set(items: list) -> list:
    """This function gets you list of unique ordered items from your origin list. It creates a copy"""
    resulting = items[::]  # this way you can make a copy of list, not just a link
    popped = 0  # how many items we popped
    for i in range(len(resulting)):
        if i == 0:
            continue
        if resulting[i - popped] in items[0:i - popped]:
            popped += 1
            pop_index = i - popped + 1  # index of an item script gonna pop out
            resulting.pop(pop_index)
    return resulting


order_list = ordered_set(items)
# here we assemble the resulting list by the order from ordered_set func
for i in order_list:
    for j in range(len(items)):
        if items[j] == i:
            res.append(items[j])

return res