Duplicate grouped entities when using 'ner' pipeline
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): 'ner' pipeline

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Have transformers 3.0.2 installed
2. Run the below code

```python
from transformers import pipeline
nlp = pipeline('ner', grouped_entities=True)
nlp('Welcome to New York')
```

## Expected behavior

We should receive `[{'entity_group': 'I-LOC', 'score': 0.9984402656555176, 'word': 'New York'}`, but instead the output has duplicated 'New York': `[{'entity_group': 'I-LOC', 'score': 0.9984402656555176, 'word': 'New York'}, {'entity_group': 'I-LOC', 'score': 0.9984402656555176, 'word': 'New York'}]`.

### The Cause of the Issue According to Me

After reading 3.0.2, I noticed that lines 1047-1049 were added.  I think this was done to fix a prior issue that caused the last named entity in the sequence to be occasionally omitted when `grouped_entities=True`.  Long story short, I think this snippet was a patch that only shifted the problem from being an occasional named entity omission to an occasional named entity duplicate.

The for-loop that precedes this snippet is inconsistent in that sometimes the last named entity gets successfully added anyway (e.g. if the `if` clause on 1025 (first iteration) or 1032 is entered on the last iteration).  In this case, there is a duplicate entry upon the calling of the new code at 1047.  On the converse, the last named entity won’t be added if the `else` clause in line 1041 is entered on the last iteration.  In this case, the final named entity correctly gets added after the new code snippet is run.

In short, there is a duplicate (I think) if (i) there is only one recognized named entity or (ii) the last named entity is one such that the tokenizer cut it up into multiple tokens.  Otherwise, there is no duplicate.

nlp(‘Welcome to Dallas’) -> duplicate 'Dallas' because 'Dallas' is the only named entity
nlp(‘HuggingFace is not located in Dallas’) -> no duplicate because there are multiple entities and the final one 'Dallas' is not tokenized into multiple tokens
nlp(‘HuggingFace is located in New York City’) -> duplicate ‘New York City’ because the final named entity 'New York City' is tokenized into multiple tokens

## Environment info

- `transformers` version: 3.0.2
- Platform: Linux-5.3.0-1031-azure-x86_64-with-glibc2.10
- Python version: 3.8.1
- PyTorch version (GPU?): 1.5.1 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no
Can you check whether this still occurs after recently merged #4987? <|||||>Thanks for the response.

Is there a special repo I have to pull from or can I just update transformers.  Assuming the latter, I just re-ran `pip install --upgrade transformers`.  After doing this, the bug persists.<|||||>No, you would have to install from source as explained in the readme.<|||||>Just cloned the repo (as directed in readme) and noticed that the issue was resolved!  Any estimation when the next update will be released?<|||||>I was still having problems similar to issues #5077 #4816 #5377 

After some debugging these are the possible reasons & fixes for wrong groupings:

Looking for feedback from maintainers on my [WIP] PR https://github.com/huggingface/transformers/pull/5970

- [ ]  [Bug Fix] add an option `ignore_subwords` to ignore subsequent ##wordpieces in predictions. Because some models train on only the first token of a word and not on the subsequent wordpieces (BERT NER default). So it makes sense doing the same thing at inference time.

    - The simplest fix is to just group the subwords with the first wordpiece. 
      - [TODO] how to handle ignored scores? just set them to 0 and calculate zero invariant mean ?
      - [TODO] handle different wordpiece_prefix ## ? possible approaches:
get it from tokenizer? but currently most tokenizers dont have a wordpiece_prefix property?
have an _is_subword(token)

- [ ]  [Bug Fix] Shouldn't group entities that are both 'B' even if they are same type 

    - (B-type1 B-type1) != (B-type1 I-type1)

- [ ] [Feature add] added option to `skip_special_tokens`. Cause It was harder to remove them after grouping.

- [ ] [Additional Changes] remove B/I prefix on returned grouped_entities 

- [ ] [Feature Request/TODO] Return indexes?

- [ ]  [Bug TODO]  can't use fast tokenizer with grouped_entities ('BertTokenizerFast' object has no attribute 'convert_tokens_to_string')
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
