Import check_inits handling of duplicate definitions.
# Improves the handling of duplicate definitions for `utils/check_inits.py`

Currently if there are duplicate definitions of import structure or type hints, the model may or not may fail depending on if the import_structure and TYPE_HINTS are duplicated in the same way. If it fails, it will show the wrong error message.

This ensures this test always fails and displays the appropriate error message.

This is particularly useful if things go wrong while merging master into a model PR after a long time.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Did you write any new necessary tests? -> N/A.


## Who can review?
 @sgugger since he first wrote the tool