Registration is cookie based, so there is no way that our system would know that a user is registered until a cookie gets written to their machine. That is why we have the 'Already registered? Enter email address' section so that your readers don't have to supply all of their information again.
The only way to avoid this is to turn off compulsory registration but then you would not be collecting new email addresses. Once your user import has been completed, a cookies still needs to be written on their machine which tells our viewer that they are already registered.