Не правильный ответ remove()

def checkio(data: list) -> list:
    flag = 0
    for i in data:
        if data.count(i) <= 1:
            data.remove(i)
        else:
            flag = True
    if flag:
        return data
    else:
        return []


print(list(checkio([10, 20, 30, 10])))

if __name__ == "__main__":
    # These "asserts" using only for self-checking and not necessary for auto-testing
    assert list(checkio([1, 2, 3, 1, 3])) == [1, 3, 1, 3], "1st example"
    assert list(checkio([1, 2, 3, 4, 5])) == [], "2nd example"
    assert list(checkio([5, 5, 5, 5, 5])) == [5, 5, 5, 5, 5], "3rd example"
    assert list(checkio([10, 9, 10, 10, 9, 8])) == [10, 9, 10, 10, 9], "4th example"
    print("It is all good. Let's check it now")

В Выводе должно быть [10, 10]

Выводит [10, 30, 10]