[exim] Avoiding duplicated E-Mails in forwarding

Hi, list!

I have a Mail server with Exim 4.75 on a Ubuntu Linux Server.
On this Server I have many accounts and some forwarders.

Let say, that I have the forwarder a@mydomain, forwarding all E-Mails to
x@mydomain, y@mydomain and z@dmydomain.
Then I have another forwarder b@mydomain, forwarding all E-Mails to x@mydomain,
k@mydomain and w@mydomain.

No problem here, but I need to have another forwarder c@mydomain that forward
all E-Mails to a@mydomain and b@mydomain.
I can't give the "real" recipient, then they can change...

Now, in this case, an E-Mail directed to c@mydomain will be delivered to
x@mydomain (duplicated, due to the entry in a@mydomain and b@mydomain),
y@mydomain, z@dmydomain, k@mydomain and w@mydomain.

Is it possible to avoid the duplicated E-Mail to x@mydomain?

Thanks a lot!
Luca Bertoncello
(lucabert@???)