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

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