Use of importlib's import_module function with modules belonging to a library
can cause some modules to be imported twice, if such a module is referenced
from sibling modules, and from __init__ in the package.  I suspect this is a
bug, or at best a nuance of packages that is quite subtle.

Easier to show with an example.  Below, the module my_lib.bar will be imported
twice.  Given the following file structure:

./scratch.py
./my_lib/__init__.py
./my_lib/foo.py
./my_lib/bar.py

And the following file contents:

./scratch.py:

	import importlib
	importlib.import_module('my_lib.bar')
	importlib.import_module('my_lib.foo')


./my_lib/__init__.py:

	import my_lib.bar

	# Or alternately
	#from . import bar


./my_lib/foo.py:

	from . import bar

	print('In foo, id(bar.baz): %s' % id(bar.baz))


./my_lib/bar.py:

	def baz():
		pass

	print('In bar, id(bar.baz): %s' % id(baz))


Running scratch.py results in my_lib.bar being imported twice:
$ echo $PYTHONPATH
.
$ python --version
Python 3.2.2
$ python ./scratch.py
In bar, id(bar.baz): 21328632
In bar, id(bar.baz): 21352240
In foo, id(bar.baz): 21352240


Replacing the calls to import_module with use of the import statement, or
__import__, or simply rearranging the order of the two calls all result in the
module my_lib.bar to be imported only once.  As does eliminating the import
statement in my_lib.__init__.

This may be a misunderstanding on my part regarding the intended use of
packages, but this behavior was quite unexpected, and rather difficult to track
down in real code.

At first glance, I thought this might be just the circular import problem (#992389) appearing in a different guise. However, if that was the case, switching to an import statement or __import__ shouldn't have made any difference.

What do you see if you put an "import my_lib.bar" *between* the two importlib calls?

I can reproduce this on tip.  What happens is that 
'importlib.import_module("my_lib.bar")' is effectively computed as:

   import my_lib
   import bar

by '_bootstrap._gcd_import'.  When '_gcd_import' goes to do the import
of 'bar' it does *not* check to see if 'bar' has already been imported
by the parent import.

Here is a patch *without* tests that fixes this.  I will add the tests
next.

Confirmed that this patch fixes the behavior shown in my original example, with 3.2.

Updated patch with tests.

Patch looks good to me.

New changeset d2504d30f259 by Meador Inge in branch '3.2':
Issue #13591: import_module potentially imports a module twice.
http://hg.python.org/cpython/rev/d2504d30f259

New changeset e8fb61a0a2d7 by Meador Inge in branch 'default':
Issue #13591: import_module potentially imports a module twice.
http://hg.python.org/cpython/rev/e8fb61a0a2d7

Thanks for the review Brett.  Fix committed.

New changeset 541f215a31f7 by Meador Inge in branch '3.2':
Issue #13591: Moving the NEWS line to the right release.
http://hg.python.org/cpython/rev/541f215a31f7

New changeset 92e94fd303d4 by Meador Inge in branch 'default':
Issue #13591: Moving the NEWS line to the right release.
http://hg.python.org/cpython/rev/92e94fd303d4