input: Write a python function to remove duplicate characters from a string.
  assert remove_duplicates("Hello World !") == "Helo Wrd!"
  assert remove_duplicates("Python Programming") == "Python rgami"
  assert remove_duplicates("Mississippi") == "Misp"
output: def remove_duplicates(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)