Well, if you want to do this in cook, just add a user data type field and click the unique property. That will ensure that each user can only add 1 entry. Furthermore, use the author wizard to auto insert the currently logged in user.
I wouldn't do it this way though. If it is in fact a user profile that you are creating, you should do it the proper way - a user profile plugin. Being a user plugin, you have immediate access to login/logout, register and more core events. This makes tasks like bailing out of authentication, registration and login etc, really simple since you are hooking your component-specific checks/routines into the standard joomla user flow. Also, setting custom session data from here is real easy.
Gez