Django 3.2 automatic AppConfig discovery breaks projects working in 3.1.8

[Manel Clos]

The described method of overriding the default admin site (​https://docs.djangoproject.com/en/3.2/ref/contrib/admin/#overriding-the-default-admin-site), is not compatible with automatic AppConfig discovery, if the app (myproject, or myapps) is already in INSTALLED_APPS.

How to reproduce

• pip install Django==3.2
• django-admin startproject myproject
• python manage.py migrate
• python manage.py createsuperuser
• Put a template in myproject/templates/admin/base_site.html
• Add myproject to INSTALLED_APPS
• python manage.py runserver works, I can access the admin site and see the changes in templates
• Follow the guide to override the default admin site, ​https://docs.djangoproject.com/en/3.2/ref/contrib/admin/#overriding-the-default-admin-site, this is, create myproject/admin.py, myproject/apps.py and modify settings.py.
• python manage.py runserver

Actual result
RuntimeError: 'myproject.apps' declares more than one default AppConfig: 'AdminConfig', 'MyAdminConfig'.`

This a regression

• pip install Django==3.1.8
• python manage.py runserver
• All ok, both templates and AdminConfig working at the same time

Workaroud

• use app-admin.py instead of apps.py when following the documentation to override the default admin site
• adjust settings.py: 'myproject.apps-admin.MyAdminConfig' in INSTALLED_APPS
• works with Django 3.2

[Mariusz Felisiak]

IMO it's a duplicate of #30402. You should customize a default admin site inside a main project directory (like described in the documentation) not in an app.

Please feel-free to send a PR with docs adjustments if you think we should make it clearer.