input: Write a function to find duplicate characters in a string.
  assert find_duplicate_characters("programming") == ['r', 'g', 'm']
  assert find_duplicate_characters("hello") == ['l']
  assert find_duplicate_characters("test") == ['t']
  assert find_duplicate_characters("aabbcc") == ['a', 'b', 'c']
  assert find_duplicate_characters("abcdef") == []
output: def find_duplicate_characters(s):
      from collections import Counter
      counts = Counter(s)
      return [char for char, count in counts.items() if count > 1]