def f(numbers):
    seen = set()
    for number in numbers:
        if number in seen:
            return number
        seen.add(number)
    return None

example_input = [64, 91, 77, 16, 22, 6, 55, 9, 56, 73, 54, 83, 14, 51, 52, 84, 33, 62, 16]
example_output = f(example_input)
print(example_output)