When a new user for some reason clicks the confirmation-link a second time, an error message is shown on the backend, without any way to redirect the user to a more descriptive page on the frontend.
-One solution would be to redirect the user to already configured login url with some extra parameter.
-Or let the admin setup a separate url to go to after the confirmation-link failed.
-Another might be to let the admin configure the url the confirmation-link goes to and then from that frontend page do a post to the backend and handle everything from there on out.