Question: I would like to do something like play.clubpenguin.com. 

What it does is that, when you visit the site, maybe using firefox or IE, if you opened a new tab or use other browser, when you visit that site again, it will show something like: "Please close the other browser that opened this page" or something like that. 

How to do this? (Its client's request)

More information: The site is Flash site

EDIT: 

Ok, i think this is a tough question, well, in general, Can this be done using php, mysql and JS?

Answer: 
try using the below code:

```
    window.onload = function(){
    if (document.cookie.indexOf("_instance=true") === -1) {
    document.cookie = "_instance=true";
    // Set the onunload function
    window.onunload = function(){
        document.cookie ="_instance=true;expires=Thu, 01-Jan-1970 00:00:01 GMT";
    };
    // Load the application
    }
    else {
    // Notify the user
    }
    };

```

The code will restrict the user to open one browser tab at a time and during browser refresh(current tab) the code will not show any alert. Copy pasting the same URL in new tab will not be allowed the user to open. for more info try [this](http://kinsey.no/blog/index.php/2009/11/15/how-to-restrict-your-web-app-to-a-single-instance/)
If you want to forbid access with 2 different logins, you can enforce a rule that lock on a given resource. 

The client IP could be one of this lockable ressource: only one session allowed for one given IP address. That would reduce the cheating to people that have multiple public IP addresses. People that shares public IP through proxy would have problem.

I don't see what other lockable resource you can use easily.