Already in

Already in redirects authenticated users who attempt to visit the login, user registration, or password reset URLs to an alternate destination instead (the /user page by default). This prevents authenticated users from getting an Access Denied error message. Optionally, the module can display a "You are already logged in" message when redirecting users.

The issue this module addresses has been fixed in Drupal 7, see #17664: user/register should show custom message instead of 403 for logged-in users.

This module borrows from code originally posted by loze and asimov.