GitLab

* fix adding special tokens when the token is already there.

* add a test

* add a test

* nit

* fix the test: make sure the order is preserved

* Update tests/test_tokenization_common.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>