In case this helps anyone, I'd been dealing with a double keychain prompt every time I'd reply to an email and then again when sending. I believe this started happening when I upgraded to my current version, 16.68.
I solved this by running Keychain Access, searching for adal (as described above), and then giving Outlook access to the two items mentioned above (com.microsoft.adalcache, Microsoft Office Credentials).
Then, doubleclick each of those items, click the Access Control tab, then click the + button and navigate to your Microsoft Outlook app, and click save changes. Do that for adalcache and MS Office Credentials. Restart outlook and, for me, the problem went away.
By the way, I first tried clicking "Allow all applications to access this item" but it didn't help. I have the other option selected (Confirm before allowing access) but with "Ask for Keychain password" unchecked.
Hope this helps someone!