Non-unique `local` in toctree
`local` field in toctree is treated as an id; therefore, it has to be unique. Because of this, in https://huggingface.co/docs/transformers/multilingual, you cannot reach the second `multilingual` page

I will work on the doc-builder that checks locals are unique;

[line here](https://github.com/huggingface/transformers/blob/master/docs/source/_toctree.yml#L32)

<img width="300" alt="Screenshot 2022-03-03 at 18 08 52" src="https://user-images.githubusercontent.com/11827707/156615355-af17d9fc-0c8b-488c-ada3-77edc923bf15.png">


Thanks for this! I think we can actually remove the first `multilingual` page since it is outdated now. On a related note, can we also remove the `custom_datasets` [page](https://huggingface.co/docs/transformers/master/en/custom_datasets) since it has also been replaced by the fine-tune for downstream tasks section?

Is this also why for section headers that share the same name, the second header cannot be reached (i.e., feature extractor [here](https://huggingface.co/docs/transformers/master/en/preprocessing#feature-extractor))?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>@mishig25 Add you added something to error when entries are not unique?