Append to list stored in table key

Hmm, my goal was to store each unique ip for given session id. So I can then analyze if different ip's were used during one http session - maybe I missed something but it's only storing IP for new session id (no key = session id in table yet) and then check if given ip is already stored in a key - if not append it to the list of ips stored as key value. At least that was my goal but maybe my logic has some flaw?

Piotr