如何确保一次只插入一次数据？

Question

这就是我的JSON的样子：

$return='{"sub":{"2":""},"postcode":"56000","slider1":"100","action":"test"}';


$data = json_decode($return, true);

这就是我获得价值的方式，

$postcode=$data['postcode'];
$budget = $data['slider1'];
$subject =$data['sub'];

var_dump($key =array_keys($data['sub']));// this prints 0=>2

foreach($key as $key1=>$key2)
{
     $key1;//prints 0
     $key2;//prints 2
}
 $key2;//prints 2

我正在插入这样的数据：

try {
  //$pdo = new PDO('mysql:host=localhost;dbname=users', $username, $password);
  //$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  include($_SERVER['DOCUMENT_ROOT'].'/config.php');
  $stmt = $pdo->prepare('INSERT INTO test (postcode,budget,subject) VALUES(:postcode,:budget,:subject)');
  $stmt->execute(array(
    ':postcode' => $postcode,
    ':budget'=> $budget,
    ':subject'=> $key2
  ));

  # Affected Rows?
  $stmt->rowCount(); // 1
} catch(PDOException $e) {
  'Error: ' . $e->getMessage();
}

上述查询将数据两次插入数据库。我检查了chrome dev工具中的网络选项卡，它显示了此文件的请求被发送两次。

有没有办法一次只插入一次数据？我的意思是有没有办法限制查询？

已编辑

AJAX

$("document").ready(function(){
          $(".form").submit(function(){
            var data = {
              "action": "test"
            };
            data = $(this).serialize();
            $.ajax({
              type: "POST",
              dataType: "json",
              url: "writedb.php", //Relative or absolute path to writedb.php file


    data: data,
                  success: function(data) {
    //.....
    }