ปรึกษา ป้องกัน การ login ซ้ำครับ กรณีไม่อยากให้ login ซ้ำครับ

<?php

	require_once('database/connection.php');

    if (isset($_POST['submit'])) { 
        $username =  $conn->real_escape_string($_POST['username']);
        $password = $conn->real_escape_string($_POST['password']);
        date_default_timezone_set('Asia/Bangkok');
        $Newtime = date('Y-m-d H:i:s');
        $Newtime1 = date('Y-m-d H:i:s');

        $sql = "SELECT * FROM `user` WHERE `username` = '".$username."'";
        $result = $conn->query($sql);
        $row = $result->fetch_assoc();

        if(!$row) {
            echo '<script> alert("ชื่อผู้ใช้งาน หรือ รหัสผ่าน ไม่ถูกต้อง") </script>';
            header('Refresh:1; url=index.php');
            exit();

        } else {
            if($row ["loginstatus"] == "1") {
                echo "<script type='text/javascript'>alert('ชื่อผู้ใช้ $username เข้าสู่ระบบอยู่แล้ว');</script>";
                header('Refresh:1; url=index.php');
                exit();

            } else {
                if(password_verify($password, $row['password'])){
                    
                    $update = "UPDATE `user` SET updated_at = '$Newtime', updated_dt = '$Newtime1',loginstatus = '1'
                        WHERE `username` = '$username' and id = '".$row["id"]."' ";
                    $runupdate = $conn->query($update);

                    
                    $_SESSION['id'] = $row['id'];
                    $_SESSION['firstname'] = $row['firstname'];
                    $_SESSION['lastname'] = $row['lastname'];
                    $_SESSION['username'] = $row['username'];
                    $_SESSION['password'] = $row['password'];
                    $_SESSION['image'] = $row['image'];
                    $_SESSION['loginstatus'] = $row['loginstatus'];
                    $_SESSION['created_at'] = $row['created_at'];
                    $_SESSION['updated_at'] = $Newtime;
                    $_SESSION['updated_dt'] = $Newtime1;
                    $_SESSION['status'] = $row['status'];

                    session_write_close();

                    
                    if ($_SESSION['status'] == 'admin') {
                    echo "<script>alert('เข้าสู่ระบบสำเร็จ'); window.location ='../cmssystem/pages/admin/dashboard/';</script>";

                    }
                    if ($_SESSION['status'] == 'member') {
                        echo "<script>alert('เข้าสู่ระบบสำเร็จ'); window.location ='../cmssystem/pages/users/dashboard/';</script>";
                        
                    }
		 }
            }
        }       		
}
	mysqli_close($conn);
?>