Remove duplicate words in docstring (#3)

Author: henriktorget

tiktoken/core.py

@@ -59,7 +59,7 @@ def encode(
     ) -> list[int]:
         """Encodes a string into tokens.
 
-        Special tokens are tokens are artificial tokens used to unlock capabilities from a model,
+        Special tokens are artificial tokens used to unlock capabilities from a model,
         such as fill-in-the-middle. So we want to be careful about accidentally encoding special
         tokens, since they can be used to trick a model into doing something we don't want it to do.