You encounter an error similar to the following when adding a domain.
(XID xxcadp) The domain "domain.tld" already exists in the userdata.
The error occurs when a domain exists within an account as either the primary domain, addon domain, or parked domain. The domain will have to be removed from the account it is on before it can be added to the intended account.
The domain will have to be removed from the account it is on before it can be added to the intended account. You can identify which account a domain belongs to with the following script.
Please note that you must replace domain.tld with the applicable domain when using the above command.