remove_duplicates
lremove_duplicates returns a new list with the 2nd, 3rd, É of any element removed
lremove_duplicates([2, 3, 4, 5, 2, 6, 5, 4]) ==>
[2, 3, 4, 5, 6]
lremove_duplicates([2, 3, 4, 5, 2, 6, 5, 4]) ==>
[2, 3, 4, 5, 6][2, 3, 4, 5, 6]