Issue #18383: Merge warnings fix from 3.5

diff --cc Lib/test/test_warnings/__init__.py
Simple merge

diff --cc Lib/warnings.py
Simple merge

diff --cc Misc/NEWS
index ee30945cdb902a927ccc8a9af9e90dbfce3e02b2,4918265b660908176a1da7783fb9317b0bdd3291..06cf8d986c94d7ddae23a6dfa8ce875f719027ce
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -2,56 -2,10 +2,59 @@@
  Python News
  +++++++++++
  
 -What's New in Python 3.5.2 release candidate 1?
 -===============================================
 +What's New in Python 3.6.0 alpha 2
 +==================================
 +
 +*Release date: XXXX-XX-XX*
 +
 +Core and Builtins
 +-----------------
 +
 +- Issue #27097: Python interpreter is now about 7% faster due to optimized
 +  instruction decoding.  Based on patch by Demur Rumed.
 +
 +- Issue #26647: Python interpreter now uses 16-bit wordcode instead of bytecode.
 +  Patch by Demur Rumed.
 +
 +- Issue #23275: Allow assigning to an empty target list in round brackets:
 +  () = iterable.
 +
 +Library
 +-------
 +
++- Issue #18383: Avoid creating duplicate filters when using filterwarnings
++  and simplefilter.  Based on patch by Alex Shkop.
+ 
 -Release date: tba
 +- Issue #23026: winreg.QueryValueEx() now return an integer for REG_QWORD type.
 +
 +- Issue #26741: subprocess.Popen destructor now emits a ResourceWarning warning
 +  if the child process is still running.
 +
 +- Issue #27056: Optimize pickle.load() and pickle.loads(), up to 10% faster
 +  to deserialize a lot of small objects.
 +
 +Tests
 +-----
 +
 +- Issue #25285: regrtest now uses subprocesses when the -j1 command line option
 +  is used: each test file runs in a fresh child process. Before, the -j1 option
 +  was ignored.
 +
 +- Issue #25285: Tools/buildbot/test.bat script now uses -j1 by default to run
 +  each test file in fresh child process.
 +
 +Windows
 +-------
 +
 +- Issue #27064: The py.exe launcher now defaults to Python 3.
 +  The Windows launcher ``py.exe`` no longer prefers an installed
 +  Python 2 version over Python 3 by default when used interactively.
 +
 +
 +What's New in Python 3.6.0 alpha 1?
 +===================================
 +
 +Release date: 2016-05-16
  
  Core and Builtins
  -----------------