spec: duplicate keyword args f(x=1, x=2) is a static error (#133)

This wording comes from google/starlark-go@2c65f9e

Fixes #21

spec.md

@@ -944,6 +944,11 @@ Once the parameters have been successfully bound to the arguments
 supplied by the call, the sequence of statements that comprise the
 function body is executed.
 
+It is a static error if a function call has two named arguments of the
+same name, such as `f(x=1, x=2)`. A call that provides a `**kwargs`
+argument may yet have two values for the same name, such as
+`f(x=1, **dict(x=2))`. This results in a dynamic error.
+
 Function arguments are evaluated in the order they appear in the call.
 <!-- see https://github.com/bazelbuild/starlark/issues/13 -->