# Your optional code here
# You can import some modules or create additional functions


def checkio(data: list) -> list:
    doublon = []
    for number in data:
        if data.count(number) > 1:
            doublon.append(number)
    return doublon


# Some hints
# You can use list.count(element) method for counting.
# Create new list with non-unique elements
# Loop over original list


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")