Tips for writing a script for Mail.app

I need to write a script to use to fine-tune triggering of a Mail.app 
rule to send out of office autoreplies to emails.

I'm mainly happy with what Mail does with its rules, except that I can't 
tell it to only send an autoreply once to each recipient.  i.e. if Bob 
sends and email on days 1 and 3 of someone's absence, only the message 
received on day 1 should get the respose.

I imagine that building an exclusion list of email addresses is the 
answer... but how?

-- 
Sara

The teeth are free at last! Fly free, young teethies!