[exim] Prevent double delivery after manualroute

Hello,
I am wondering if someone could give me a piece of advise how to avoid
double deliveries. The situation I have is a server running exim (old)
and a server to which exim routes messages for some users (new,
migration ongoing). I use the manualroute router that makes the
routing decision based on the presence of the local_part in a text
file. The old server has users set in /etc/passwd and groups in
/etc/aliases
The problem is, when an email arrives to the old server, which is
addressed to the user as well as a group where that user belongs. Then
exim figures out it has to use the manualroute user because he is in
the list, but then delivers a duplicate after expanding the mail
group. How can this be avoided?
Regards,
Patryk