[ `TokenizationUtils`] Fix `add_special_tokens` when the token is already there (#28520)

* 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>

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

src/transformers/tokenization_utils_base.py

@@ -943,14 +943,15 @@ class SpecialTokensMixin:
                     isinstance(t, (str, AddedToken)) for t in value
                 ), f"Tokens {value} for key {key} should all be str or AddedToken instances"
 
-                to_add = set()
+                to_add = []
                 for token in value:
                     if isinstance(token, str):
                         # for legacy purpose we default to stripping. `test_add_tokens_tokenizer` depends on this
                         token = AddedToken(token, rstrip=False, lstrip=False, normalized=False, special=True)
-                    if str(token) not in self.additional_special_tokens:
-                        to_add.add(token)
-                if replace_additional_special_tokens:
+                    if not replace_additional_special_tokens and str(token) in self.additional_special_tokens:
+                        continue
+                    to_add.append(token)
+                if replace_additional_special_tokens and len(to_add) > 0:
                     setattr(self, key, list(to_add))
                 else:
                     self._additional_special_tokens.extend(to_add)

tests/test_tokenization_common.py

@@ -4139,3 +4139,28 @@ class TokenizerTesterMixin:
                                 _test_added_vocab_and_eos(
                                     EXPECTED_ADDED_TOKENS_DECODER, self.tokenizer_class, new_eos, tmp_dir_4
                                 )
+
+    def test_special_token_addition(self):
+        for tokenizer, pretrained_name, kwargs in self.tokenizers_list:
+            with self.subTest(f"{tokenizer.__class__.__name__} ({pretrained_name})"):
+                # Create tokenizer and add an additional special token
+                tokenizer_1 = tokenizer.from_pretrained(pretrained_name)
+                tokenizer_1.add_special_tokens({"additional_special_tokens": ["<tok>"]})
+                self.assertEqual(tokenizer_1.additional_special_tokens, ["<tok>"])
+                with tempfile.TemporaryDirectory() as tmp_dir:
+                    tokenizer_1.save_pretrained(tmp_dir)
+                    # Load the above tokenizer and add the same special token a second time
+                    tokenizer_2 = tokenizer.from_pretrained(pretrained_name)
+                    tokenizer_2.add_special_tokens({"additional_special_tokens": ["<tok>"]})
+                    self.assertEqual(tokenizer_2.additional_special_tokens, ["<tok>"])
+
+                    tokenizer_2.add_special_tokens({"additional_special_tokens": ["<tok>", "<other>"]})
+                    self.assertEqual(tokenizer_2.additional_special_tokens, ["<tok>", "<other>"])
+                    tokenizer_2.add_special_tokens({"additional_special_tokens": ["<other>", "<another>"]})
+                    self.assertEqual(tokenizer_2.additional_special_tokens, ["<other>", "<another>"])
+
+                    tokenizer_2.add_special_tokens(
+                        {"additional_special_tokens": ["<tok>"]},
+                        replace_additional_special_tokens=False,
+                    )
+                    self.assertEqual(tokenizer_2.additional_special_tokens, ["<other>", "<another>", "<tok>"])