autodoc: duplication warning on documenting aliased object

[tk0miya]

Describe the bug
autodoc: duplication warning on documenting aliased object

To Reproduce

# example.py
from io import StringIO

# index.rst
.. autoclass:: example.StringIO
.. autoclass:: io.StringIO

Removing everything under '_build'...
Running Sphinx v4.0.0+/dfdc7626b
making output directory... done
[autosummary] generating autosummary for: index.rst
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 1 source files that are out of date
updating environment: [new config] 1 added, 0 changed, 0 removed
reading sources... [100%] index
docstring of _io.StringIO:1: WARNING: duplicate object description of _io.StringIO, other instance in index, use :noindex: for one of them
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] index
generating indices... genindex done
writing additional pages... search done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 1 warning.

The HTML pages are in _build/html.

Expected behavior
No warning

Your project
N/A

Screenshots
N/A

Environment info

• OS: Mac
• Python version: 3.9.4
• Sphinx version: HEAD of 4.0.x
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No

[tk0miya]

I noticed the example is not good. Both example.StringIO and io.StringIO are aliases of _io.StringIO. So they're surely conflicted.

It would be better to not emit a warning for this case:

.. autoclass:: _io.StringIO
.. autoclass:: io.StringIO

The former one is a canonical name of the io.StringIO. So this should not be conflicted.