Problem with Registering User on Submit

Hi,
I have a specific problem when registering a user using a form. The problem is that I also have a username and an email address from another user in 2 separate fields which are brought into the form using jQuery (I use this info as the destination email address from a contact form, this is dynamic and changed between users already registered on the site.)
So basically when I press submit the new user is registered and receives the email and the form is submitted. However, it seems that during the validation process of the form the other username and email address of the already registered user is being detected in the form and the final screen is completely white with the “The username is associated with another user” warning. It does not flow through to the results page as required and the newly registered user does not get the required feedback of their successful registration.
Is there any way to have another username and email address of an already registered user contained in a field (for use in the emails sent at submission) and still register a new user without the warning screen?
Thanks for your help in this..