Session Management (Limited session per one client 's browser )

Hi all members.

This is the firsrt times I join into Microsoft Comunication. And I hope that I am helped by you.

First of all, I have been developing  a website . My manager need a limit login per client's browser.

For example :

In traditional, your account is simplesoftware, you login into my site at IE browser.
After that, you can login in another browser (such as FF,Chrome).And everything is okie.

My manager doesn't need above logic. He think that users have to have 1 session in runtime.  
And If users try to login at 2 (or more) browser , a banned screen would throw : 1 user / 1 session.

And this is a my code :

------------------------------MY CODE------------------------------
[page.aspx]
public partial class page : System.Web.UI.Page {

    protected bool Login(string userName) {

        System.Collec ...