Getting problem in storing data in Mysql databse using php

I am creating a Posting system like facebook. But when I am going to store my data in MySQL database, this problem is got. Every time I reload the page, my last post is being saved again and again automatically. Here is my code

<?php

    $con= mysqli_connect("sql103.ezyro.com", "ezyro_24157515", "fq3850whf75pj5", "ezyro_24157515_Leash");  

   if(isset($_POST['Launch'])) {

        $title= $_POST['title'];
        $desc= $_POST['desc'];
        $date=date("Y-m-d h:i:sa");
        $users= $_SESSION['username'];

        $q= "INSERT INTO `leashes` (`id`, `user_name`, `title`, `description`, `date_time`)
                VALUES('','".$users."', '".$title."', '".$desc."', '".$date."')";
        $r= mysqli_query($con, $q);
        if($r){
        echo 'Done';
        }else{
        echo $q;
        }
        }

?>

I have a login system from where I get the user's name. Please help me to overcome this problem.