Added ValueError for duplicates in list of added tokens
Very small addition to raise an error if the list of tokens passed to `add_tokens` contains duplicates. This otherwise raises cryptic errors down the line. Happy to update it to `Warning` if someone believes there's any reason for duplicates to be allowed here.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=h1) Report
> Merging [#1296](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/391db836ab7ed2ca61c51a7cf1b135b6ab92be58?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1296/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1296   +/-   ##
=======================================
  Coverage   84.72%   84.72%           
=======================================
  Files          84       84           
  Lines       12591    12591           
=======================================
  Hits        10668    10668           
  Misses       1923     1923
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1296/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl91dGlscy5weQ==) | `91.48% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=footer). Last update [391db83...a951585](https://codecov.io/gh/huggingface/transformers/pull/1296?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok, great, thanks @danai-antoniou!