While working with a customer, I witnessed some confusion where the user

tried to log in, got redirected back to the Log In page, and then

thought they hadn't successfully logged in. This was occurring because

they were navigating to the Log In page, clicking Log In in the

navigation bar, and ending up back to the Log In page with a redirect

URL set back to the same page. Worse, if clicking Log In from the Log

Out page, users would get logged out immediately after logging in.
To avoid these situations, we now redirect away from the Log In page if

the user is already logged in. We only do this after we check for all

the SSO/client-side login flows, so that we don't disrupt those.
If the destination page would be the Log In or Log Out pages, we

normalize it back out to root URL for the server/Local Site.  This

avoids these loops, and helpfully ensures users won't get immediately

logged out when logging in.
Django has a mechanism to do these sort of checks, but it doesn't handle

the redirect-to-login case. Actually, it does, but it handles it via

raising an exception. Our approach is more tailored to our needs.

Tested logging in and redirecting.
Tested visiting the page while already logged in, and testing without

a login redirect, with a suitable redirect, and with a redirect loop.
Performed the same test when clicking the Log In link from the Log Out

page.
Unit tests pass.