SF #1685563, MSVCCompiler creates redundant and long PATH strings

If MSVCCompiler.initialize() was called multiple times, the path
would get duplicated.  On Windows, this is a problem because the
path is limited to 4k.  There's no benefit in adding a path multiple
times, so prevent that from occuring.  We also normalize the path
before checking for duplicates so things like /a and /a/ won't both
be stored.

Will backport.
diff --git a/Misc/NEWS b/Misc/NEWS
index 9cbdc3a..408de56 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -1,4 +1,4 @@
-__init+++++++++++
++++++++++++
 Python News
 +++++++++++
 
@@ -200,6 +200,8 @@
 Library
 -------
 
+- Patch #1685563: remove (don't add) duplicate paths in distutils.MSVCCompiler.
+
 - Added a timeout parameter to the constructor of other protocols
   (telnetlib, ftplib, smtplib and poplib). This is second part of the
   work started with create_connection() and timeout in httplib, and