Apache tomcat version: 6.0.20
If user is already logged, and he tries to login again from login page, the j_security_check is not available error is encountered. Is it normal behaviour or I have to do something?
Actually I have different user roles for accessing different pages, and when access to a page is denied to a particular user, I want to redirect him to login page, where he can login with corresponding credentials.
This behavior is normal: the servlet spec only lays-out the procedure for container-managed authentication (i.e. login) when the user requests a protected resource and the user has not already provided credentials. All other scenarios are left undefined, including yours.
If you want to capture "forbidden" conditions, you can use <error-page> mappings in your WEB-INF/web.xml to send the user anywhere you want, including a login page. Just remember that the container will only accept a login after the above conditions are true, so you may have to log the user out first (by terminating the user's session).
What I might recommend is a "forbidden" page that says "You don't have access to this resource. If you'd like to log-in as a different user to access it, please click [HERE]" where [HERE] is a link to a servlet that terminates the user's session and then redirects to the resource the user was trying to access. This will cause the container to request authentication (i.e. present the login form), verify the credentials, and send the user to the desired resource.
If you are using a container (and webapp) along with version 3.0 of the servlet specification, there is a new HttpServletRequest.login() method that can be used to programmatically log a user into your webapp. You might be able to use that instead of terminating the session and doing all those redirects... instead, you could collect the username and password yourself and then ask the container to do the login for you.