Delete unique elements from a list

Question

I faced some problem with solving the next problem:

We have a list of elements (integers), and we should return a list consisting of only the non-unique elements in

Answer 1

Would it not be easier to generate a new list?

def unique_list(lst):
    new_list = []
    for value in lst:
        if value not in new_list:
            new_list.append(value)
    return new_list

lst = [1,2,3,1,4,5,1,6,2,3,7,8,9]
print(unique_list(lst))

Prints [1,2,3,4,5,6,7,8,9]