Using Generics to Multiply Collection

I have to write a method that takes arguments List and int n.

Method should multiply content of given List in the same List. If n = 0, than List shoud be empty. If n = 1, List should be the same as before.

For example:

List<Integer> list = Arrays.asList(1,2,3);
multiply(list, 2);
System.out.println(list);

---

desired output:

[1,2,3,1,2,3]

Method signature can not be changed:

public static void multiply(List<?> list, int n) {

}

I tried this:

public static void multiply(List<?> list, int n) {

        List<? super Object> copy = new ArrayList<>();

        if (n == 0) {
            list.clear();
        }

        for (int i = 1; i <= n; i++) {
            copy.addAll(list);         
        }

        list.clear();
        list.addAll(copy); // this is not allowed
    }

Thank you for advices!