input: Write a python function to remove duplicate characters from a string.
  assert remove_duplicates("DaSSmmoonnd") == 'aDoSdmn'
  assert remove_duplicates("hello") == 'helo'
  assert remove_duplicates("programming") == 'progamin'
output: def remove_duplicates(input_string):
      seen = set()
      result = []
      for char in input_string:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)