While migrating a client to cloudflare recently with a zone that was not active on cloudflare, it eventaually became apparent that the domain had previously been setup on cloudflare, and that cloudflare DNS servers would respond to queries using the zone data from the previous account.
Obviously cloudflare has no way of telling which account is valid, so to contine with the first account makes sense.
However there is a way to resolve this situation where a zone exists in more than one account.
If a new account is created and tries to use a domain zone that has already been created on cloudflare , then the second account should have some method of verification, such as a DNS TXT record, which if it passes, deactivates the domain from the previous account, and activates it on the new account, if it does not pass, then the previous account would stil be active.
This has the advantage of not impacting valid existing accounts (the test will fail unless they have access to DNS), and allowing valid new accounts to complete setup of the zone and test against the zone before going live and changing the NS records for the domain to point at cloudflare.