find array intersections

(3)  Given two lists of integers, write a function that returns a list
that contains only the intersection (elements that occur in both lists)
of the two lists. The returned list should only contain unique integers,
no duplicates.

For example, [4, 2, 73, 11, -5] and [-5, 73, -1, 9, 9, 4, 7] would
return a list containing [-5, 4, 73] in no particular order.