写一个函数，将一个字符串作为输入，并返回其中唯一的单词列表。忽略任何标点符号和数字。
"This is a test. This is only a test."
```python
def unique_words(string):
  words = string.split()
  unique = []
  for word in words:
    clean_word = ''.join(e for e in word if e.isalpha())
    if clean_word.lower() not in unique:
      unique.append(clean_word.lower())
  return unique
``` 

Output: `['this', 'is', 'a', 'test', 'only']`