input: Write a function to return the second smallest element in a list.
  assert find_second_smallest([1, 2, -8, -2, 0, -2])==-2
  assert find_second_smallest([1, 1, -0.5, 0, 2, -2, -2])==-0.5
  assert find_second_smallest([2,2])==None
output: def find_second_smallest(numbers):
      unique_numbers = list(set(numbers))
      if len(unique_numbers) < 2:
          return None
      unique_numbers.sort()
      return unique_numbers[1]