A type warning for html_use_opensearch is shown twice

[tk0miya]

Describe the bug
A type warning for html_use_opensearch is shown twice.

To Reproduce

# conf.py

Running Sphinx v3.5.0+/332d80ba8
making output directory... done
WARNING: The config value `html_use_opensearch' has type `int', defaults to `str'.
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 1 source files that are out of date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
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... WARNING: html_use_opensearch config value must now be a string
done
writing output... [100%] index
generating indices... genindex py-modindex done
writing additional pages... search opensearch done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 2 warnings.

The HTML pages are in _build/html.

Expected behavior
It's enough to warn only once.

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.9.1
• Sphinx version: HEAD of 3.x
• Sphinx extensions: No
• Extra tools: No

Additional context
No