Golang stream requests

Question

I was wondering how to solve a situation like this:

I've built a simple login/logout server using Golang, when using Postman to send POST request to signup user, it works fine.

But when I spam the server with Postman, it sign up user without going through the validation (checking if same email exist etc.) or should I say that the request has been processed almost simultaneously that's why the validation is not working. How can I solve a problem like this?

Thanks.

I have done some research and some say using websocket and net/http, but I don't think that solve the problem.