In both shops

Two shops each have a list of items. Write in_both(a, b) that returns a list of the items that appear in both, each item once, in any order.

in_both(["apple", "bread", "milk"], ["milk", "eggs", "apple"])  ->  ["apple", "milk"]   (or ["milk", "apple"])
in_both(["apple"], ["bread"])                                   ->  []

def in_both(a, b):
    ...