DuplicatedKeysError when loading tweet_qa dataset
Hi, we've just merged the PR with the fix. The fixed version of the dataset can be downloaded as follows:
```python
import datasets
dset = datasets.load_dataset("tweet_qa", revision="master")
```
When loading the tweet_qa dataset with `load_dataset('tweet_qa')`, the following error occurs: 

`DuplicatedKeysError: FAILURE TO GENERATE DATASET !
Found duplicate Key: 2a167f9e016ba338e1813fed275a6a1e
Keys should be unique and deterministic in nature
`
Might be related to issues #2433 and #2333

- `datasets` version: 1.17.0
- Python version: 3.8.5
