[exim] UNSEEN option

Good morning list (depending where you are globally)

I have had a request to "cc" all mails to a specific user to a user at
another company.

Exchange seems to have issues (well lots of them).

I have decided to complete the request on the Exim gateway (relay)
server.

I have been reading the documentation and see
That the unseen option can be used to fulfil this requst.

<snip>
The unseen option can be used to cause copies of messages to be
delivered to some other destination, while also carrying out a normal
delivery. In effect, the current address is made into a "parent" that
has two children - one that is delivered as specified by this router,
and a clone that goes on to be routed further.
<snip>

Any idea's how this can be implemented?

Basically all mails to user@??? must be delivered BUT also
cc'd to user1@???.

Many thanks

Regards,
Tom