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