wp_insert_post is creating duplicate posts from Safari

I’m creating custom post types from the front end with a form. It works fine in Chrome and Firefox, however from Safari duplicate posts are created. This is part of the code

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

$tags = $_POST['post_tags'];

$new_post = array(
    'post_title'    => $title,
    'post_content'  => $description,
    'post_category' => array($_POST['cat']),
    'tags_input'    => array($tags),
    'post_status'   => 'publish',etc.
    'post_type' => 'website'
);

global $current_user;
get_currentuserinfo();

$user_name = strtolower($current_user->user_login);  //custom caps will default to lowercase anyway
$pid = wp_insert_post($new_post);

update_post_meta($pid, "s2member_ccaps_req", $user_name, true);

wp_redirect( get_permalink($pid) );
exit();

I’ve viewed the $pid using var_dump and it returns a valid ID, the code works fine in Chrome and FF, so what else can this be? Any tests I can run?