[Dovecot] How does sieve's vacation keep track of duplicates?

[ben at electricembers.net]

I see the .dovecot.lda-dupes file that is created, but it doesn't seem 
like just plain text, and it doesn't seem to include strings of the 
senders, just the recipient. But I know the vacation functionality is 
clever enough to keep track of who it's already autoreplied to.

On a related note, I'm curious about other smarts built in to sieve's 
vacation responder -- what other things does it check, like Precedence 
header?

  Benjamin