Fix duplicating words words. (GH-6296)

Most of them have been added in 3.7.
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 0746fcf..7b0971a 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -1153,7 +1153,7 @@
    *module*.
 
    The *name_of_module* argument can specify (as a string or tuple thereof) what
-   module(s) an API could be defined in in order to be detected as a public
+   module(s) an API could be defined in order to be detected as a public
    API. One case for this is when *module* imports part of its public API from
    other modules, possibly a C backend (like ``csv`` and its ``_csv``).