防止将重复数据输入到mysql数据库中

我正试图使我的电子邮件订阅服务拒绝数据库中已经存在的电子邮件，以使用户不会两次订阅同一电子邮件.这是我所拥有的，但是不起作用，有什么想法吗?

I'm trying to make my email subscription service reject emails that already exist within my database so users don't subscribe the same email twice. this is what I have but its not working, any ideas?

<?php 
    if(!isset($_POST['submit'])) 
        exit();

    $vars = array('email');
    $verified = TRUE;
    foreach($vars as $v) {
        if(!isset($_POST[$v]) || empty($_POST[$v])) {
            $verified = FALSE;
        }
    }   
    if(!$verified) {
        echo "<p style='color:white; margin-top:25px;'>*Email required*</p>";
        exit();
    }
    $email = $_POST['email'];
    if($_POST['submit']) echo "<p style='color:white; margin-top:25px;'>*Check your inbox*        </p>";

    // Create connection
    $con=mysqli_connect("mysql.host","user","password","dbname");

    // Check connection
    if (mysqli_connect_errno($con))
    {
        echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }

    $sql="INSERT INTO emails (email)    VALUES  ('$_POST[email]')";
    if (!mysqli_query($con,$sql))
    {
        die('Error: ' . mysqli_error($con));
    }

    $query = mysql_query("SELECT * FROM emails WHERE email='$email'",($con));
    if(mysql_num_rows($query) != 1)
    {
        echo "email already exists";
        // redirect back to form and populate with 
        // data that has already been entered by the user
    }


    mysqli_close($con);
?>