session expired for already registered users

When a user accidentally tries to register twice (using the same email) there should be a friendly hint but my CB gives out a weird browser message, see screenshot. Am I missing a setting in CB? The message appears weird to the user (me in that case) becasue I have cookies enabled and what session could be expired:


When I click ok I gets even more unpleasent:

"
When I now reload the page I get a message that I have successfully registered and "You are already signed up with this username and password..." Which is true but here utterly confussing.

How can I prevent that from happening?

Attempting to register with an already registered email address won't throw that error. It instead throws a validation error that email is already in use. What you're seeing is due to the spam/spoof checking failing due to the security token sent with the HTML form being expired and attempted to be reused. Did they press back on their browser and try to register again? Do you have caching of any kind enabled? Caching is the common culprit of that error.