[exim] Doubled headers with exiscan

I'm using exiscan with spamassassin which set X-Spam-Level: header. I've found
the problem if this header already exists in incoming message. How can I
delete header in ACL. I'm afraid it is impossible, but I think the system
filter is called after ACL is working, so after adding the doubled header.

Of course the message could come from my other server, so I use following ACL:

  # Omit if already scanned
  accept  condition     = ${if eq {${hmac{md5}{EXISCAN_SA_SECRET}
{SA$body_linecount}}}\
                               {$h_X-Exiscan-SA-Signature:} {1}{0}}

-- 
 .''`.    Piotr Roszatycki, Netia SA
: :' :    mailto:Piotr_Roszatycki@netia.net.pl
`. `'     mailto:dexter@debian.org
  `-