in about:logins, when I try to manually create/edit an entry which duplicates (as in, has same URL and username) an existing one, this action may not be completed, yet no error is shown. The click on Save simply does nothing.

The case when such functionality might be handy is local development: you don't necessarily have all your admin passwords in sync for different projects that you work on on your machine, and you probably don't want to remember all the passwords either. Ideally, I would be able to add additional notes on such saved passwords, for easier identification, but even without them, I'd rather cycle through a couple options than have to look at all my chat logs and emails.

But even if this ability to save multiple passwords is not desired, I'd expect at least an error message to be shown for a disallowed action as opposed to nothing at all hapenning.

Are you on an updated Nightly? This was fixed a few days ago. Re-open if it happens on the latest Nightly.