"""\
    Non-unique Elements

    You are given a non-empty list of integers (X). For this task, you should
    return a list consisting of only the non-unique elements in this list.
    To do so you will need to remove all unique elements (elements which are
    contained in a given list only once). When solving this task, do not
    change the order of the list. Example: [1, 2, 3, 1, 3] 1 and 3 non-unique
    elements and result will be [1, 3, 1, 3].

    Input: A list of integers.

    Output: The list of integers.

    Example:
     checkio([1, 2, 3, 1, 3]) == [1, 3, 1, 3]
     checkio([1, 2, 3, 4, 5]) == []
     checkio([5, 5, 5, 5, 5]) == [5, 5, 5, 5, 5]
     checkio([10, 9, 10, 10, 9, 8]) == [10, 9, 10, 10, 9]

    How it is used: This mission will help you to understand how to manipulate
    arrays, something that is the basis for solving more complex tasks. The
    concept can be easily generalized for real world tasks. For example: if
    you need to clarify statistics by removing low frequency elements (noise).

    Precondition:
     0 < len(data) < 1000
"""
def checkio(data: list) -> list:
    dataNU = []
    for i in data:
        if data.count(i) > 1:
            dataNU.append(i)
    return dataNU

print(checkio([1, 2, 3, 1, 3]))
print(checkio([1, 2, 3, 4, 5]))
print(checkio([5, 5, 5, 5, 5]))
print(checkio([10, 9, 10, 10, 9, 8]))

# nice
