is it possible, if the current user logedin in another browser and try to login again on a browser then show a "already logedin"alert

this is for a live class site, as they don’t want to provide MULTIPLE Login from a user

Yeah Try this …
This will show a box in a page when the user have logged in 2 devices, browser etc in real Time

First you should create a new web module (.jsw) file and rename it as - realtime

Code for the web module file →

// Filename: backend/realtime.jsw (web modules need to have a .jsw extension)

import { publish } from 'wix-realtime-backend';

export function publishMessage(message) {
 const channel2 =  { name: 'messaging' };
 const payload = {message};
 return publish(channel2, payload);
}

Code for the login page →

import wixUsers from 'wix-users';
import { publishMessage } from 'backend/realtime';


$w.onReady(function () {
$w('#submitButtton').onClick((event) => {    // login button
 let email = $w('#email').value;     //email input
 let password = $w('#password').value;    //password input
    wixUsers.login(email, password)

    wixUsers.onLogin((user) => {
 let userId = user.id;
 let message = userId;
 return publishMessage(message);
     });
    });
  });

Code for the page where the box should be shown or the user will be logged out->

#box1 - the box to show ()

import * as realtime from 'wix-realtime';
import wixUsers from 'wix-users';

$w.onReady(function () {
$w('#box1').hide();
 const channel2 = { name: 'messaging' };
    realtime.subscribe(channel2, type);

});

async function type({ payload }) {
 if (payload.message === wixUsers.currentUser.id) {
   wixUsers.logout();
 //or 
 if ($w('#box1').hidden) {
            $w('#box1').show('fade');
            setTimeout(() => $w('#box1').hide('fade'), 10000);
        } else {
            setTimeout(() => type({ payload }), 10000);
        }
    }
}