Preventing multiple threads for the same routine and the same user?

Question

I’m developing a web app on ASP.NET Core MVC and some routines are called by clicking a button on the web page.

It turns out you can push this button multiple times, so I need some bloqueie/suspenda/... unwanted threads.

I tried to do it with this code (C#), where the variable User is a session variable, corresponding to the user:

[HttpPost]
public async Task<IActionResult> SendOrdForm(
    int id, 
    ProdListViewModel model, 
    IFormCollection form
)
{
    UserModel user = HttpContext.Session.GetJson<UserModel>("User");
    if (user.AwaitingOrderSubmit)
        while (true); // blocks thread or aborts it... What's the best option?

    user.AwaitingOrderSubmit = true;
    HttpContext.Session.SetJson("User", user);
    
    // ... the rest of the long and time-expensive routine ...
    user.AwaitingOrderSubmit = false;
    HttpContext.Session.SetJson("User", user);
    return RedirectToAction("");
}

How can I make this code only executable once thread and user, by code on this or another routine?