Now i want that a user can make only a single session with the proxy server i.e. multiple user can not login with a single User id
Shoot the ones who share their credentials.
Don't shoot the ones who don't
Short of that? Not that I can see.
You realize if you limit the proxy sessions to one per user, they get one tab, one window regardless of browser and checking email?
Forget it, you'd have to shut down any port connections that extend past the proxy server to do that.
Download a file?
You better know how to curl.
Are you sure this is what you want to do?