I own my iZotope product outright, but when I enter my email address and serial number I am being told that this product is already registered to another account.

This message appears when the email address being used to activate your software does not match the email address to which the product is registered. Make sure you’re entering the email address you used the first time you activated your product. If you are having trouble locating your original activation information, please contact iZotope Customer Care for further assistance. You can submit an online support ticket here.