Thats what I would also would expect, but even If wp_config debug is false, there are these php-error messages instead some info for the user. And it is never merged as you described.
Configuration Panel multiple registrations are allowed.
For me as a user I would at first expect that the user never sees error messages, even if he is registered yet. And the merge into to lists does not work.
It worked some month ago when I created that all on my website. Now it has this bug.