According to Microsoft's knowledgebase, even using a rule will reply only
once per Outlook session. From
http://support.microsoft.com/kb/311107 :
"The Rules Wizard rule to 'reply using a specific template' is designed to
send the reply only one time to each sender during a session. This
prevents Outlook from sending repetitive replies to a sender from whom you
receive multiple messages.
During a session, Outlook remembers the list of users to whom it has
responded. When you restart Outlook, this list is deleted and the rule is
reset to start again for each sender".