input: Write a function to remove duplicate characters from a string.
  assert remove_duplicate("Hello World") == "Helo Wrld"
  assert remove_duplicate("aabbcc") == "abc"
  assert remove_duplicate("abcabcabc") == "abc"
  assert remove_duplicate("Python Programming") == "Python rgami"
output: def remove_duplicate(s):
      result = []
      seen = set()
      for char in s:
          if char not in seen:
              result.append(char)
              seen.add(char)
      return ''.join(result)