$errormessage}, If user already exist in database then add him again ?

[Alfred]

Hello, i am designing my website in a way that i want to process if the user is already exist in database at checkout form then create a dummy user with random email and address so the user can process the payment because in my case i can define that user,

let me explain : 

normaly the user at checkout enter his email and full name & pass etc.. , what i want to do is when the $errormessage get fired about existing email i want that user to complete to order confirmation page  nomatter what, if that is not possible.
i want then to generate a dummy email to replace the existing email so he can process to order confirmation page.

What i have so far to generate the ddummy data is this using javascript to get dummy data from an API:

    //Get API
    fetch('https://randomuser.me/api/?results=100')
        .then(res => res.json())
        .then(data => {
            let author = data.results;
            author.forEach(function (lists) {
                document.getElementById('inputEmail').value = lists.login.username+="@gmail.com";
            });
        });

 

i hope i was clear explaining my situation.

thank you very much .