Registration form validation problem

[Maggy29]

Joomla 2.5.6
VM 2.0.10

When a client registers on checkout with a username or email address that's already being used by another client, an error message is displayed, for example "This email is already registered".
Instead of the user being forced to change this data in the registration form, he/she has to click the "Add/Edit billing address information" button to edit the form.
In my opinion this is not very user friendly. Is there a way to change this?

As far as I can see, edit_address.php only checks for required fields; as soon as they are entered, the email and username check is done in the standard Joomla registration module.

Anyone any ideas? Or is there a configuration option I missed?