input: Write a python function to find the first duplicate character in a string.
  assert find_duplicate_char("programming") == 'r'
  assert find_duplicate_char("hello") == 'l'
  assert find_duplicate_char("abcd") == 'No duplicates found'
  assert find_duplicate_char("aabbcc") == 'a'
output: def find_duplicate_char(s):
      seen = set()
      for char in s:
          if char in seen:
              return char
          seen.add(char)
      return 'No duplicates found'