DuplicatedKeysError of NewsQA dataset
Thanks for reporting, @StevenTang1998.

I'm fixing it. 
After processing the dataset following official [NewsQA](https://github.com/Maluuba/newsqa), I used datasets to load it:
```
a = load_dataset('newsqa', data_dir='news')
```
and the following error occurred: 
```
Using custom data configuration default-data_dir=news
Downloading and preparing dataset newsqa/default to /root/.cache/huggingface/datasets/newsqa/default-data_dir=news/1.0.0/b0b23e22d94a3d352ad9d75aff2b71375264a122fae301463079ee8595e05ab9...
Traceback (most recent call last):        
  File "/usr/local/lib/python3.8/dist-packages/datasets/builder.py", line 1084, in _prepare_split
    writer.write(example, key)
  File "/usr/local/lib/python3.8/dist-packages/datasets/arrow_writer.py", line 442, in write
    self.check_duplicate_keys()
  File "/usr/local/lib/python3.8/dist-packages/datasets/arrow_writer.py", line 453, in check_duplicate_keys
    raise DuplicatedKeysError(key)
datasets.keyhash.DuplicatedKeysError: FAILURE TO GENERATE DATASET !
Found duplicate Key: ./cnn/stories/6a0f9c8a5d0c6e8949b37924163c92923fe5770d.story
Keys should be unique and deterministic in nature

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/dist-packages/datasets/load.py", line 1694, in load_dataset
    builder_instance.download_and_prepare(
  File "/usr/local/lib/python3.8/dist-packages/datasets/builder.py", line 595, in download_and_prepare
    self._download_and_prepare(
  File "/usr/local/lib/python3.8/dist-packages/datasets/builder.py", line 684, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/usr/local/lib/python3.8/dist-packages/datasets/builder.py", line 1086, in _prepare_split
    num_examples, num_bytes = writer.finalize()
  File "/usr/local/lib/python3.8/dist-packages/datasets/arrow_writer.py", line 524, in finalize
    self.check_duplicate_keys()
  File "/usr/local/lib/python3.8/dist-packages/datasets/arrow_writer.py", line 453, in check_duplicate_keys
    raise DuplicatedKeysError(key)
datasets.keyhash.DuplicatedKeysError: FAILURE TO GENERATE DATASET !
Found duplicate Key: ./cnn/stories/6a0f9c8a5d0c6e8949b37924163c92923fe5770d.story
Keys should be unique and deterministic in nature
```