Fixed fixed a a doubled doubled word word in in PEP PEP 457 457.

pep-0457.txt

@@ -247,7 +247,7 @@ no value was passed in for it when the function was called?
 
 The obvious solution: add a new singleton constant to Python
 that is passed in when a parameter is not mapped to an argument.
-I propose that the value be called called ``undefined``,
+I propose that the value be called ``undefined``,
 and be a singleton of a special class called ``Undefined``.
 If a positional-only parameter did not receive an argument
 when called, its value would be set to ``undefined``.