The uniqueness of a domain zone is an important (if not fundamental) requirement of every DNS provider. Generally speaking, the DNS infrastructure of a DNS provider should be designed to allow only one specific instance of the same domain zone.
It means that if you registered the domain
example.com with foocompany and you are hosting the DNS with barcompany, if I create an account in barcompany I should not be able to create a domain in my account called
If I could, then it means than the company MUST have a way to determine who is the authoritative ownwer. But this makes things more complicated, and there is no reason to follow this way.
However, I may be able to create an account with bazcompany and publish there a zone for
example.com regardless I'm the registrant or not. But this has no real effect.
Because you are managing the domain, as long as the name server for the domain are pointing to barcompany, the domain will resolve with the configuration you created in your barcompany account.