more than one Stamdard Primary DNS zone providing redundancy

I have a Domain controller "ABC" on Machine1. Machine 2 is hosting "Standard Primary DNS Zone" (read/write) for ABC domain stored in text file. It is said that with "Standard Primary DNS Zone" you can create only one read/write zone. But on Machine3 i am able to create another "Standard Primary DNS Zone" for ABC domain, and it provides read/write facility when Machine 2 is not available. How can it be possible? I think i am missing something about "Standard Primary DNS zone". Kindly clarify.

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