Recognize when a account login at the same time

Hi,

I've an web app that use a handler for different functions e.g. for the login.
So when a user login I create a cookie for:
Call: ../login.api?User=test&pw=test
Code behind: Session["ID"]=result_from_sql

This works fine but now I have a problem:
When one user account is active and on an other place a user try to login with the some account, the working one should be informed about.
As the login is done by the handler I've no idea how to do this.
Maybe is a way to work with Global.asax to manage it but I've never used it so is just an idea...

Has somebody a solution how to do something like this?

Thanks,

Andre