我一直得到这个:完整性约束违反，而试图验证电子邮件是否存在于数据库

我正在创建一个用户管理系统。我可以编辑用户。我可以创建用户。我可以验证邮件的格式是否正确。然而，我的问题是与验证相同的电子邮件是否存在于数据库中。我一直得到这个错误:哎呦，未能运行查询:SQLSTATE[23000]:完整性约束违反:1062 duplicate du champ 'Markr@fun.com' pour la clef 'email'。代码如下所示。第一个是用于向数据库存储信息的表单。第二个是在按下提交按钮后运行的脚本。

<?php
require("../scripts/connect.php");

if(empty($_SESSION['user']))
{
    header("Location: ../hound/login.php");

    die("Redirecting to ../hound/login.php");
}
$query_parm = array(
':id' => $_GET['id']
 );

 $query = "
SELECT
*
FROM users 
WHERE 
id = :id
";

try
{
 $stmt = $db->prepare($query);
 $stmt->execute($query_parm);
}
catch (PDOException $ex)
{
die("Failed to run query: " . $ex->getMessage());
}
  $rows = $stmt->fetchAll();

  ?>

  <form action="../scripts/edit_users.php" method="post">
<?php foreach($rows as $row): ?>
Username:<br />
<b><?php echo htmlentities($row['username'], ENT_QUOTES, 'UTF-8'); ?></b>
<br /><br />
<input type="hidden" name="id" value="<?php htmlentities($row['id'],  ENT_QUOTES, 'UTF-8'); ?>">
First Name:<br />
<input type="text" name="first_name" value="<?php echo `enter code he  htmlentities($row['first_name'], ENT_QUOTES, 'UTF-8'); ?>" />
<br /><br />
Last Name:<br />
<input type="text" name="last_name" value="<?php echo htmlentities ($row['last_name'], ENT_QUOTES, 'UTF-8'); ?>" /> 
<br /><br />
E-Mail Address:<br />
<input type="text" name="email" value="<?php echo htmlentities($row     ['email'],ENT_QUOTES,'UTF-8'); ?>" /> 
<br /><br />
Password:<br />
<input type="password" name="password" value="" /><br />
<br /><br />
<input type="submit" value="Update User" />
<a href="../scripts/users.php">Back</a><br />
 <?php endforeach; ?>
 </form>

这是当submit被按下时运行的脚本。

<?php

require("common.php");
if(empty($_SESSION['user']))
{
    header("Location: ../hound/login.php");

    die("Redirecting to ../hound/login.php");
}
if(!empty($_POST))
{
if (!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL))
{
    die("Please enter a valid email address...");
}
if($_POST['email'] !=$_POST['email'])
{
    $query_email = "
         SELECT email
         from users
         where
         email = :email
    ";
    $query_goes = array(
    ':email' => $_POST['email']
    );
    try
    {
        $stmt = $db->prepare($query_email);
        $result = $stmt->execute($query_goes);
    }
    catch (PDOException $ex)
    {
        die("Failed to run query: " . $ex->getMessage());
    }
    $row = $stmt->fetch();
    if($row) 
    {
        die("That email is already in use...");
    }
}
}

  $array_value = array(
        ':email' => $_POST['email'],
        ':first_name' => $_POST['first_name'],
        ':last_name' => $_POST['last_name'],
        ':id' => $_POST['id']
   );

    $query = "UPDATE users 
        SET 
        email = :email,
        first_name = :first_name, 
        last_name = :last_name
        WHERE
          id = :id
        ";

       try
    {
        $stmt = $db->prepare($query);
        $result = $stmt->execute($array_value);
    }
    catch(PDOException $ex)
    {
        die("Ouch, failed to run query: " . $ex->getMessage());
    }

    header("Location: users.php");

    die("Redirecting to users.php");
   ?>