MY_Controller and login-authentication misunderstanding probably

[eluser]blitzneo[/eluser]
cideveloper yup, that's what I said =) I created a library called MY_Controller.php in application/core/My_Controller, like InsiteFX said.
My question is, if a user is already logged in, I think he/she shouldn't be able to see for example login, signup controllers again, soooo maybe that controllers should be not extended by that MY_Controller library, right ? Or just check if they are already logged in, redirect to the index ?
Maybe my last post wasn't clear enough, sorry.