Rails session_id is not unique

We have been experiencing an issue in our environment (Jruby 1.4, Rails 2.3.5, Tomcat 5.5, Java 1.6) where duplicate session_ids are being created in the sessions table. We are assuming the only way this can happen is when 2 sessions are created at the exact same time on 2 different machines (we have 10 app servers running with config.threadsafe! on). Has anyone encountered this issue? It seems the “randomness” of the session creation is not unique enough and doesnt account for multiple machines (tomcat uses the ip of the machine as well i believe).