Write a method `rem()` that takes as input a list containing, possibly, duplicate values
and returns a copy of the list in which one copy of every duplicate value removed.

```
> rem([4])
[]

> rem([4, 4])
[4]

> rem([4, 1, 3, 2])
[]

> rem([2, 4, 2, 4, 4])
[2, 4, 4]

```

How do I go about doing this? Do I make a new list that contains the duplicates?