input: Write a function to remove duplicate characters from a given string. If the given string is empty then return an empty string.
  assert remove_duplicate_characters("bufferoverflow")=="bufrOvw"
  assert remove_duplicate_characters("hello")=="helo"
  assert remove_duplicate_characters("aaabbbccc")=="abc"
  assert remove_duplicate_characters("")==""
  assert remove_duplicate_characters("abcdef")=="abcdef"
output: def remove_duplicate_characters(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)