How do I reroute a logged-in user after page access denied at user/login?

My Drupal 6 site has the login form at /user/login. If I'm logged in already and I click it, I'm sent to the Access Denied page (though the login URL stays the same).

I'd like to send such users somewhere else. Anywhere else. /user would be fine. Is there anything I can hook into to achieve this?