input: Write a function to find the index of the first element that appears more than once in a list.
  assert find_multi_element_index(['a', 'b', 'a', 'c', 'a']) == 2
  assert find_multi_element_index(['x', 'y', 'z', 'x']) == 3
  assert find_multi_element_index(['1', '2', '3', '4']) == None
output: def find_multi_element_index(lst):
      seen = {}
      for index, element in enumerate(lst):
          if element in seen:
              return index
          seen[element] = True
      return None