First Repeated Value

Write first_duplicate(values: list[int]) -> int that returns the first value whose second appearance is reached while walking the list from left to right, or -1 when nothing repeats. For 3 1 4 1 5 3 the answer is 1, because the second 1 comes before the second 3. Keep a set of the values seen so far and use its add() method. Do NOT write a main block or any top-level test code; the tests construct the calls. Input: one line of space-separated integers (possibly empty); the tests print the value your function returns.