First duplicate

To find the first repeat, remember every value you have seen so far. Walk the list; the first value that is already in your memory is the answer. A set is the natural place to keep that memory.

Write first_duplicate(items) that returns the first item which has appeared earlier in the list, or None if every item is different. "First" means the item whose second appearance comes earliest, so for [1, 2, 2, 1] the answer is 2, not 1.

first_duplicate(["milk", "bread", "milk", "bread"])  ->  "milk"
first_duplicate([1, 2, 2, 1])                        ->  2
first_duplicate([1, 2, 3])                           ->  None

def first_duplicate(items):
    ...