input: Write a python function to find and print keys that have duplicate values in a dictionary.
  assert find_duplicate_keys({'a': 1, 'b': 2, 'c': 1, 'd': 3})==['a', 'c']
  assert find_duplicate_keys({'x': 5, 'y': 6, 'z': 5, 'w': 5})==['x', 'z', 'w']
  assert find_duplicate_keys({'k': 1, 'l': 2, 'm': 3})==[]
output: def find_duplicate_keys(d):
      value_to_keys = {}
      for key, value in d.items():
          if value in value_to_keys:
              value_to_keys[value].append(key)
          else:
              value_to_keys[value] = [key]
      
      duplicates = []
      for keys in value_to_keys.values():
          if len(keys) > 1:
              duplicates.extend(keys)
      
      return duplicates