If two people buy a particular domain at the same time from different providers, what will happen?
If the two requests are made through different registrars, whichever EPP request is processed by the registry first will succeed, and the second one will fail. How this failure is handled will depend on the registrar.
If the two requests are made through the same registrar, the results will depend on the registrar. In practice, the first request should "win", but some registrars may not have tested this case, and weird things may happen.