Normally you wont offer your clients the permission to create new websites at all as you never know who owns a domain. they can always add a domain that is woned by someone else then. Normally sites are created by the admin and ispconfig prevents that the client changes the domain.
That's fine. But does it make sense that Client b may add the domain of client a within it's sites? On one side for sure, if I logically split the functionality of email and web on _one_ domain across two users, but from my point of view this is are more rare szenario than a client (in this case a) owns a domain (in this case test.com) for email as well as for web, dns etc.?!
Thats not that rare. For example are some companys offering free subdomains of their domain(s) for websites to their clients while they use the main domain for email for their own company. We had this problem several times with ISPConfig 2.
But I agree that we might add some kind of optional lock to prevent the adding of domains where the email domain is owned by another client.