Fix duplicating words words. (GH-6296)

Most of them have been added in 3.7.
diff --git a/Doc/library/asyncio-protocol.rst b/Doc/library/asyncio-protocol.rst
index 004cac8..ef64416 100644
--- a/Doc/library/asyncio-protocol.rst
+++ b/Doc/library/asyncio-protocol.rst
@@ -339,7 +339,7 @@
    control of the receive buffer.
 
    .. versionadded:: 3.7
-      **Important:** this has been been added to asyncio in Python 3.7
+      **Important:** this has been added to asyncio in Python 3.7
       *on a provisional basis*!  Treat it as an experimental API that
       might be changed or removed in Python 3.8.
 
@@ -450,7 +450,7 @@
 ------------------------------------------------------
 
 .. versionadded:: 3.7
-   **Important:** :class:`BufferedProtocol` has been been added to
+   **Important:** :class:`BufferedProtocol` has been added to
    asyncio in Python 3.7 *on a provisional basis*!  Consider it as an
    experimental API that might be changed or removed in Python 3.8.
 
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:
diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
index c1b164e..8d91f4e 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -2209,8 +2209,8 @@
       :meth:`utcoffset` is transformed into a string of the form
       ±HHMM[SS[.uuuuuu]], where HH is a 2-digit string giving the number of UTC
       offset hours, and MM is a 2-digit string giving the number of UTC offset
-      minutes, SS is a 2-digit string string giving the number of UTC offset
-      seconds and uuuuuu is a 2-digit string string giving the number of UTC
+      minutes, SS is a 2-digit string giving the number of UTC offset
+      seconds and uuuuuu is a 2-digit string giving the number of UTC
       offset microseconds.  The uuuuuu part is omitted when the offset is a
       whole number of minutes and both the uuuuuu and the SS parts are omitted
       when the offset is a whole number of minutes.  For example, if
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``).