How to know user is already login at another browser in firebase - reactjs

Question

I have reactjs admin panel for my project admin module. I have requirement that is if 2 user have same userId and password then both of them can not login in same time. If User1 already has logged into admin panel then User2 should not be login. I have get Id token of logged in user and saved it into database now I wanted to check whether the user is already login or not.
I dont have any idea how to prevent user to login in admin panel if other user is already logged in.
Please show some light on code and suggest me right path to fulfill my requirement
Here is my code:
//AUTHENTICATION
var loggedIn = false;
var activeSession = true;
if (Config.firebaseConfig.apiKey) {
firebase.auth().onAuthStateChanged(function (user) {
// Retriving ID Token
var Auth = firebase.auth();
var idToken = Auth.currentUser.getToken();
firebase.database().ref('activeSession').push({
idToken:idToken,
userEmail : user.email,
userId: user.uid,
isUserLoggedIn : activeSession
});
firebase.auth().verifyIdToken(idToken).then(function(decodedToken) {
console.log(decodedToken);
var uid = decodedToken.uid;
console.log(uid);
// ...
}).catch(function(error) {
// Handle error
});
}).catch(function(error) {
// Handle error
console.log(error);
});
if (user) {
// User is signed in.
console.log("User is signed in " + user.email);
if (Config.adminConfig.allowedUsers != null && Config.adminConfig.allowedUsers.indexOf(user.email) == -1) {
//Error, this user is not allowed anyway
alert("The user " + user.email + " doens't have access to this admin panel!");
firebase.auth().signOut();
} else {
loggedIn = true;
displayApp();
}
} else {
// No user is signed in.
console.log("No user is signed in ");
loggedIn = false;
displayApp();
if (window.display) {
window.display();
}
}
} else {
// No user is signed in.
console.log("No user is signed in, step 1 ");
loggedIn = false;
displayApp();
if (window.display) {
window.display();
}
}
function displayApp() {
if (loggedIn) {
ReactDOM.render(
<Admin />,
document.getElementById('root')
);
} else {
ReactDOM.render(
<Login />,
document.getElementById('root')
);
}
}
displayApp();