Fix duplicating words words. (GH-6296)

Most of them have been added in 3.7.
diff --git a/Doc/library/contextvars.rst b/Doc/library/contextvars.rst
index 1e0987c..abd0d5f 100644
--- a/Doc/library/contextvars.rst
+++ b/Doc/library/contextvars.rst
@@ -253,7 +253,7 @@
         addr = writer.transport.get_extra_info('socket').getpeername()
         client_addr_var.set(addr)
 
-        # In any code that we call is is now possible to get
+        # In any code that we call is now possible to get
         # client's address by calling 'client_addr_var.get()'.
 
         while True: