input: Develop a function to locate the second minimum value in a list.
  assert second_minimum([1, 2, -8, -2, 0, -2])==-2
  assert second_minimum([1, 1, -0.5, 0, 2, -2, -2])==-0.5
  assert second_minimum([2,2])==None
output:     
      unique_numbers = list(set(numbers))
      if len(unique_numbers) < 2:
          return None
      unique_numbers.sort()
      return unique_numbers[1]
  