WARNING: duplicate object description

[mezhaka]

A Python class with type annotation produces a warning: WARNING: duplicate object description during the sphinx-build if the sphinx.ext.napoleon extension is used. This used to not produce any warnings on version 1.8.5 of sphinx, but produces the warnings in version 3.4.3.

To Reproduce
Steps to reproduce the behavior:

#!/usr/bin/env bash
set -e
set -x

mkdir repro && cd repro
mkdir -p docs/source


cat << EOF > generate_warning.py
class Test:
    """
    Attributes:
        attr: a doc
    """

    attr: bool
EOF


cat << EOF > docs/source/conf.py
import os
import sys
sys.path.insert(0, os.path.abspath('../../'))

extensions = [
    'sphinx.ext.napoleon',
]

source_suffix = '.rst'

# The master toctree document.
master_doc = 'index'
EOF


cat << EOF > docs/source/index.rst
.. toctree::

   apidoc/modules
EOF

# I use conda to create environemnts, but if you don't create a virtual env with
# sphinx 3.4.3 instead of the following block
cat << EOF > environment.yml
name: sphinx-duplicate-warning
channels:
  - conda-forge
  - nodefaults

dependencies:
  - sphinx==3.4.3
EOF

conda env create

Then activate the environment and run sphinx build:

cd repro
conda activate sphinx-duplicate-warning

sphinx-apidoc --force --separate -o docs/source/apidoc . && sphinx-build -M html "docs/source" "build"

Expected behavior
No warning

Your project
repro.zip

Environment info

• OS: MacOS 10.15.7
• Python version: 3.8.6
• Sphinx version: 3.4.3
• Sphinx extensions: sphinx.ext.napoleon
• Extra tools: [e.g. Browser, tex or something else]