[Exim] exim filter (doing something once)

Hi,

I'm currently using exim system filter to get copies of some
messages:

if $h_from: contains xyz@???
then
        unseen deliver other@??? errors_to postmaster@???
endif

But when xyz@??? sends one mail to few people (4 for example)
then I'm getting 4 copies of one and the same message.

How to get only one copy of this message? Is this possible at system
filter level?

--
Arkadiusz Miśkiewicz    CS at FoE, Wroclaw University of Technology
arekm@???   AM2-6BONE, 1024/3DB19BBD, arekm(at)ircnet, PLD/Linux