I 100% agree with RedFilter's generic answer, but wanted to post some relevant code for ASP.NET MVC specifically.

You can use the [Post/Redirect/Get (PRG) Pattern][1] to solve the double postback problem.  

Here's an graphical illustration of the problem:

![Diagram][3]

What happens is when the user hits refresh, the browser attempts to resubmit the last request it made.  If the last request was a post, the browser will attempt to do that.

Most browsers know that this isn't typically what the user wants to do, so will automatically ask:

> **Chrome** - 
The page that you're looking for used information that you entered. 
Returning to that page might cause any action you took to be repeated. 
Do you want to continue?  
> **Firefox** - To display this page, Firefox must send information that will repeat any action (such as a search or order confirmation) that was performed earlier.  
> **Safari** - 
Are you sure you want to send a form again? 
To reopen this page Safari must resend a form.  This might result in duplicate purchases, comments, or other actions.  
> **Internet Explorer** - 
To display the webpage again, the web browser needs to
resend the information you've previously submitted.
If you were making a purchase, you should click Cancel to
avoid a duplicate transaction. Otherwise, click Retry to display
the webpage again.



But the PRG pattern helps avoid this altogether by sending the client a redirect message so when the page finally appears, the last request the browser executed was a GET request for the new resource.

Here's a [great article on PRG][2] that provides an implementation of the pattern for MVC.  It's important to note that you only want to resort to a redirect when an non-[idempotent][4] action is performed on the server.  In other words, if you have a valid model and have actually persisted the data in some way, then it's important to ensure the request isn't accidentally submitted twice.  But if the model is invalid, the current page and model should be returned so the user can make any necessary modifications.

Here's an example Controller:



    [HttpGet]
    public ActionResult Edit(int id) {
        var model = new EditModel();
        //...
        return View(model);
    }

    [HttpPost]
    public ActionResult Edit(EditModel model) {
        if (ModelState.IsValid) {
            product = repository.SaveOrUpdate(model);
            return RedirectToAction("Details", new { id = product.Id });
        }
        return View(model);
    }

    [HttpGet]
    public ActionResult Details(int id) {
        var model = new DetailModel();
        //...
        return View(model);
    }

[1]: http://en.wikipedia.org/wiki/Post/Redirect/Get
[2]: https://www.stevefenton.co.uk/Content/Blog/Date/201104/Blog/ASP-NET-MVC-Post-Redirect-Get-Pattern/
[3]: http://i.imgur.com/eUcYWkd.png
[4]: http://en.wikipedia.org/wiki/Idempotence

I have a purchase page and I don't want the user to be able to refresh the page and resubmit the form once they get to the 'order complete' page because it automatically sets them up in our system via database values and charges their card via paypal (only want these to happen ONCE)... I have seen some sites that say 'Don't hit refresh or you will get charged twice!' but that is pretty lame to leave it open to possibility, what's a good way to only allow it to be submitted once or prevent them from refreshing, etc?

PS: I saw a few similar questions: https://stackoverflow.com/questions/1646812/php-stop-a-form-from-being-accidentally-reprocessed-when-back-is-pressed and https://stackoverflow.com/questions/665399/how-do-i-stop-the-back-and-refresh-buttons-from-resubmitting-my-form but found no satisfactory answer... an ASP.NET MVC specific answer would be ideal too if there is a mechanism for this.

EDIT: Once they click submit it POSTS to my controller and then the controller does some magic and then returns a view with an order complete message, but if I click refresh on my browser it does the whole 'do you want to resend this form?' that is bad...

What is a good method for preventing a user from submitting a form twice?

Instead of attaching an extra listener to the form submit, you can use the [`submitHandler`][1]:

> Callback for handling the actual submit when the form is valid.  

The default function behaves like this:



    submitHandler: function(form) {
      form.submit();
    }

> **Note**: `form` refers to a DOM element, this way the validation isn't triggered again.

Just pass in a `submitHandler` to `validate()` and add the code to disable the button like this:



    $("form").validate({
      submitHandler: function(form) {
        $(':submit', form).attr('disabled', 'disabled');
        form.submit();
      }
    });

Just be careful doing anything to the form inside of the `submitHandler` callback because it's easy to [trigger a case of infinite recursion][2]


### Here's a demo in Stack Snippets:






    $("form").validate({
      submitHandler: function(form) {
        $(':submit', form).attr('disabled', 'disabled');
        form.submit();
      }
    });

    // show message on invalid submission
    $("form").on("invalid-form.validate", function (event, validator) {
      var errors = validator.numberOfInvalids();
      if (errors) {
        alert('There is a problem with ' + errors + ' fields.');
      }
    });



    <script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.js"></script>
    <script src="//cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.13.1/jquery.validate.js"></script>

    <form >
      <input type="text" name="Name" required /><br/>
      <input type="text" name="Email" required /><br/>
      <input type="submit" value="Submit"/>
    </form>




[1]: http://jqueryvalidation.org/validate/#submithandler
[2]: http://jqueryvalidation.org/documentation/#link-too-much-recursion

Using [jQuery Validate][1], is there a way to determine if a form is valid *without revalidating* the form.

Per this question on [how to add a 'submitHandler' function when using jQuery Unobtrusive Validation?][2], I'm listening to the `invalid-form.validate` event to show the user a custom error when the form has been validated and has errors.

Per this question on how to [prevent double click on submit][3], I'm disabling the submit button once the form has been submitted, but only if the form is valid, so the user can take corrective action and resubmit once the form is cleaned up.

The problem is now `invalid-form.validate` fires twice.

Since the `invalid-form.validate` event fires before my form submission handler, I'll already know the number of errors.  Does the [`.valid()`][4] method persist the number of errors somewhere that I can check against that value instead of revalidating the form.  

Here's an example in stack snippets.  Hit submit before filling in the required fields and you'll see two error messages.





    $("form").validate();

    // show message on invalid submission
    $("form").on("invalid-form.validate", function (event, validator) {
      var errors = validator.numberOfInvalids();
      if (errors) {
        alert('There is a problem with ' + errors + ' fields.');
      }
    });

    // prevent double click form submission
    $('form').submit(function () {
      if ($(this).valid()) {
        $(':submit', this).attr('disabled', 'disabled');
      }
    });




    <script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.js"></script>
    <script src="//cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.13.1/jquery.validate.js"></script>

    <form >
      <input type="text" name="Name" required /><br/>
      <input type="text" name="Email" required /><br/>
      <input type="submit" value="Submit"/>
    </form>



I could save them to the the element in my `invalid-form.validate`, but this won't update when the form is valid and also couples my double click code very heavily with my custom error code.  If I take out the custom error code at some later date, I'll have to rewrite the double click prevention. 

Any ideas?


[1]: http://jqueryvalidation.org/
[2]: https://stackoverflow.com/a/4799723/1366033 
[3]: https://stackoverflow.com/a/6435544/1366033
[4]: http://jqueryvalidation.org/valid/