[exim] Disable deduplication

Scenario: I want to archive customer e-mail. Right now I am sending
copies through a router via "manualroute" and "unseen" which works
perfectly fine.

The only problem occurs when mails have multiple recipients like Bcc.
The recipient of the mails must be rewritten to the same address (i.e.
foobar@???) for the mailarchive to work – I use headers_add for
adding "X-Envelope-To:" in order for the mail archive to distinguish
them later.

The problem is that Exim recognizes these mails as the same and either
sends one mail with multiple recipients or (when I add "max_rcpt = 1")
sends multiple mails - but these mails are all the same, they only have
the "X-Envelope-To:" of the latest redirection.

Debugging the process shows:

"[duplicate, would not be delivered]"

As far as I understand this is expected behaviour and only within pipes
mails are treated differently.

tl;dr: how can I force Exim to actually create single and unique mails
instead of just ignoring duplicates?