Multiple Instances of the WP Uploaded in Metaboxes

/* my html markup looks like */
<input id="upload_image" type="text" size="36" name="upload_image" value="" />
<button class="upload_image_button" type="button"><?php _e('Upload Image');?></button>
<br /><span>Enter an URL or upload an image for the banner.</span>

/*my script looks like */

jQuery(document).ready(function($) {
var formfield;
$('.upload_image_button').click(function() {
    formfield = $(this).prev('input');
    tb_show('', 'media-upload.php?type=image&TB_iframe=true');
    return false;
});

window.original_send_to_editor = window.send_to_editor;
window.send_to_editor = function(html){
    if (formfield) {
        fileurl = $('img',html).attr('src');
        formfield.val(fileurl);
        tb_remove();
        formfield = ''; //reset formfield to null
    } else {
        window.original_send_to_editor(html);
    }
};

}); //end doc-ready