[Exim] how to avoid dupes ?

Sirs, who can try to explain me how can i avoid dublicate messages in my
in/out archives ?
At this moment, any messages with $sender_address_domain = mydomain are being
copied to outarchive, all other messages are being copied to inarchive.
That's work fine, but then all messages are being checked for X-SPAM-STATUS:
Yes, and when yes -- delivered to spam@ and then finish. But messages,
addressed to spam@ are being cheked with system filter again, and
$sender_address_domain as far as i can understand will be detected (and he
is) as mydomain, because this message was
readdressed by system-filter.

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Michael V. Sokolov, SA/Helpdesk