Kazoo does the same thing as the carriers. If you try create an IP auth device in callflow and then try create another IP auth device with the same IP (even if it is in a different account), Kazoo will give you a "SIP IP already in use" error.
Kazoo does not care about inbound IP from carriers as long as the IP is added to the ACL. It knows which account to route the call to based on the DID number. So again, that is the same as what most carriers seem to do.