Can anyone explain the behavior of a list and the list.remove(x) method in a loop?

Question

Python

Hello! I solved the problem on checkiO, according to the conditions of the problem, a list of numbers (data) is given, you need to return a list of non-unique numbers. It would seem that everything is simple:

def checkio(data: list) -> list:
    for d in data:
        if data.count(d) < 2: #или == 1, поведение не меняется
            data.remove(d)
    return data

But this does not work, for example, for the list [1, 2, 3, 4, 5] the function returns [2,4] (and if you continue, even values ​​are returned), although the list should be empty.
As a result, I decided through .append, but found the following code in the solutions:

result = data.copy()
for n in data:
    if(data.count(n) == 1):
        result.remove(n)
return result

And I didn’t really understand why it works with a copy of the list and does not work with the list itself passed to the function?