How to allow only one session per user

[mamiakimo]

I have been assigned with a task to restrict multiple simultaneous usages of a silverlight application for a single user. Ideally each time user opens new application, previous instance should receive some sort of notification and shut down.Silverlight application consumes data using WCF services exposed via REST endpoint.Currently every call to the service has username and password as parameters. This was good because we were having stateless services. There are some apps which use these services already, and I am looking for the least painful solution.Thanks