input: Write a function to find duplicate elements.
  assert find_duplicate([1, 2, 3, 4, 5, 1, 5]) == 1
  assert find_duplicate([10, 20, 30, 10, 40, 50]) == 10
  assert find_duplicate([100, 200, 300, 400]) == None
output: def find_duplicate(lst):
      seen = set()
      for num in lst:
          if num in seen:
              return num
          seen.add(num)
      return None