在页面刷新时添加用户？

Hey guys i am learning php and inserting records into my database.

I am confused as to why on page refresh it keeps adding records into my database before I submit the form?

    <?php

include 'core/functions/connect.php';

// escape variables for security
$username = mysqli_real_escape_string($con, $_POST['username']);
$password = mysqli_real_escape_string($con, $_POST['password']);
$first_name = mysqli_real_escape_string($con, $_POST['first_name']);
$last_name = mysqli_real_escape_string($con, $_POST['last_name']);
$email = mysqli_real_escape_string($con, $_POST['email']);

$sql="INSERT INTO users (username, password, first_name, last_name, email )
VALUES ('$username', '$password', '$first_name', '$last_name', '$email')";


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

}
echo "1 record added";

?>




<form action="" method="post">
    <ul>
        <li>
            Username*:<br>
            <input type="text" name="username">
        </li>
        <li>
            Password*:<br>
            <input type="password" name="password">
        </li>
        <li>
            Password again*:<br>
            <input type="password" name="password_again">        
        </li>
        <li>
            First name*:<br>
            <input type="text" name="first_name">
        </li>   
        <li>
            Last name:<br>
            <input type="text" name="last_name">
        </li> 
        <li>
            Email address*:<br>
            <input type="text" name="email">
        </li>  
        <li>
        <input type="submit" name="submit" value="Register">
        </li>            
        </ul>
</form>

I guess I am missing something here but a pointer would be great so i can understand where I am going wrong?

EDIT: I have tried to resolve the issue by the suggest methods below but yes it has stopped inserting on refresh but not submitting data?

Please check the POST array to see if the form has been submitted. Try doing everything you want after the form submit using:

if(isset($_POST['submit'])) // name of your submit button
    // your processing code
}