input: Write a function to find the index of the first duplicate element in a list.
  assert find_first_duplicate([5, 7, 9, 3, 1, 3])==4
  assert find_first_duplicate([1, 2, 3, 4, 5, 6])==-1
  assert find_first_duplicate([2, 4, 2, 6, 7, 8, 4])==2
output: def find_first_duplicate(lst):
      seen = set()
      for i, value in enumerate(lst):
          if value in seen:
              return i
          seen.add(value)
      return -1