Form resubmit issue on page refresh – while on preview page in wordpress

I run wordpress as CMS. I have put together a simple multi-part form which works pretty well and allows to post from front-end. It is a two part form where the first part is where he submits data and second step where he gets a preview of data he just submitted. The submitted data creates a nice draft post in wp as expected.

Here is a first chunk of code within the form’s page template, which displays the form. In the lower part of the code you will note a PREVIEW SECTION (read the comments within code) which displays the data that was just submitted. All of this works really well, I have even been able to retrieve an image, after the submit.

     <?php
            global $wpdb;
            $this_page  =   $_SERVER['REQUEST_URI'];
            $page       =   $_POST['page'];
            if ( $page == NULL ) { ?> 

        <form method="POST" id="test_form" name="test_form" enctype="multipart/form-data" action="">

        <div>LOCATION : <input type="text" name="location" id="location"/></div>

        <div>DESCRIPTION : <textarea id="details" cols="80" rows="10 maxlength="600" name="details" rows="20"></textarea></div>

        <div>UPLOAD IMAGE : <input type="file" name="loc-image" id="loc-image" tabindex="25" /></div>


        <input type="hidden" value="1" name="page" />
        <input type="hidden" name="action" value="post_action" />
        <input type="submit" name="submit" id="submit" value="PROCEED"/>
        </form>

        <?php 
        } else if ( $page == 1 ) { ?> 

        <?php include_once('validate_first_step.php'); ?>
        <?php if (isset($_POST['submit']) && (!empty($error))) { ?>

                    <h3>Submission Failed.  Errors highlighted below.</h3><br/>
                    <a href="javascript:history.go(-1)">GO BACK</a><br/><br/>
                    <?php echo $error . '</br>';
                    } else { ?>


<?php echo 'h2>'.'YOUR SUBMISSION IS ACCEPTED. PREVIEW.'. '</h2>';?>

    <?php  //PREVIEW SECTION OF THE FORM BEGINS 
    $location=$_POST['location'];
    $description=$_POST['details'];
    ?>

    <?php
    echo 'Location : ' . $location . '</br>';
    echo 'Details  : ' . $description . '</br>'; 
    ?>
    <?php echo wp_get_attachment_image( $newupload,'medium' ); ?>

    <?php //PREVIEW ENDS
    }
    }
    ?>

This is how the form is processed. Actual code is pretty complex, I have just put here that was necessary to deliver the idea. Pretty standard stuff for inserting in wordpress.

if( 'POST' == $_SERVER['REQUEST_METHOD'] &&  !empty( $_POST['action'] ) && $_POST['action'] == "post_action") {


    // Do some minor form validation to make sure there is content
    if (isset($_POST['submit'])) {
            $error = "";

        if ($_POST['details'] != null) {
            $description = trim(strip_tags($_POST['details']));
        } else {
            $error .= 'Put description.</br>';
        }

        if ($_POST['location'] != null) {
            $location = trim(strip_tags($_POST['location']));
        } else {
            $error .= 'Put location.</br>';
        }

    }  



            if (empty($error)) {   
            $new_post = array(   //insert form inputs, set var and define array
            'post_title'    =>  $location, 
            'post_content'  =>  $description,
            'post_status'   =>  'draft',
            'post_author'   =>  '2',    
            'post_type' =>  'post'  
            // assigning tags and categories are no issue
            );

            $pid = wp_insert_post($new_post);


                    //attachment helper function    
                    function insert_attachment($file_handler,$post_id,$setthumb='false') {

                    if ($_FILES[$file_handler]['error'] !== UPLOAD_ERR_OK){ return __return_false(); 
                    } 
                    require_once(ABSPATH . "wp-admin" . '/includes/image.php');
                    require_once(ABSPATH . "wp-admin" . '/includes/file.php');
                    require_once(ABSPATH . "wp-admin" . '/includes/media.php');

                    $attach_id = media_handle_upload( $file_handler, $post_id );

                    //set post thumbnail
                    if ($setthumb) update_post_meta($post_id,'_thumbnail_id',$attach_id);
                    return $attach_id;
                    }


                    //INSERT OUR MEDIA ATTACHMENTS
                    if ($_FILES) {
                    foreach ($_FILES as $file => $array) {
                    $newupload = insert_attachment($file,$pid);
                    }
                    } 

        } //end of if empty error
    }

ISSUE: Here is the issue that I noticed while testing the form. After the form submit when I am in the PREVIEW SECTION, data is displayed correctly as expected BUT when I REFRESH the page the same data is submitted again into wordpress, creating a duplicate draft post.

THINGS TRIED : I am aware of a redirection solution and able to employ wp_redirect , but redirection is not what I am looking at as I need to show them the preview. I have spent a few hours looking for solution and alternatives and still doing it.

REQUEST : Please let me know how I can stop this from happening. I cannot expect people not to click refresh, they will basically do whatever they want. So instead of a javascript notification I would like to perform this using server-side php. As it may perform even if the javascript is disabled, as will the rest of my code. Basically I need a solution that simply ignores the refresh, after the submit. Please suggest me a code or a hint to right direction that can make this possible. Many thanks.

UPDATE : As suggested by @user2758004 I tried the comparision method and it worked. I have put the following code at top of the processor.

$location=$_POST['location'];

$args = array(
        'post__not_in'=> array($id),
        'post_type' => 'post',
        'post_status' => array('publish','pending','draft','future','private'),
        'meta_query' => array(
        'relation' => 'AND',

        array(
        'key' => 'location',
        'value' => $location,
        'compare' => '='
        ),
    )
);

Now within the processor I check if the meta_value of the meta_key ‘full_name’ already exists. In case it does, the form displays an error and simply stops the process. Hope it helps someone.

    $existingMeta = get_posts( $args );
        if(!empty($existingMeta)){ 
    $error .= 'Bummer, ' .$location. ' is already in database.</br>';
        return;

} 
        else { 
                  //check for rest of the errors
    }