[Exim] Filters: coping with repeated lines

I'm trying to filter a mailing list using Sender: and have
noticed messages slipping through. I'm assuming it's because,
in those messages, there are two Sender: lines. Does this
effectively mean I would have to use "contains" rather than
"begins" everywhere? And is this guaranteed to work?
(I.e. will exim check the second line?)

(I think begins should work on all lines or else "begins"
is meaningless in multi-line situations.)

My apologies if this is documented, I didn't see it on a
semi-cursory look through the filter spec...

Paul