This is the scenario:
I have an account in WHMCS. This account already activated a domain in plesk and can already login in plesk.
This account buys another domain, a new set of username/password is created and sent to the user via email. This set of username/passoword does not work, since the customer account already exists.
How can I detect already existing customers and send them the appropriate login information (or a reminder they already have the, or nothing, anything but a wrong information is fine).