I think you have the know configuration for what you want to do. The whole thing about Domino Joyrnalling is that the email is tagged as Journalled and that tag is sent with the email as it goes from one server to another.
There are two settings that Domino journalling can do –
<b>From Domino Help</b>
Specifies the location of the Mail Journaling database. Choose one:
Copy to local database – (default) The Router copies each journaled message to a database on the local server. If it does not already exist, Domino creates a local Mail Journaling database on the server. If the Configuration Settings document applies to multiple servers, Domino creates a unique Mail Journaling database on each server.
Send to mail-in database – The Router copies each journaled message and sends it to a specified mail-in database. The specified database must already exist and must have a Mail-in database document in the Domino Directory. The mail-in database used for journaling may be on any Domino server, including the local server. Specify the mail file where journaled messages are to be sent in the Mail Destination field. When using a mail-in database for journaling, be sure to encrypt messages when adding them to the database. To encrypt messages sent to a mail-in database, enable encryption on the Administration tab of the Mail-in database document.
<b>You can also add an external address – like a third party SMTP Journal device</b>
If you specified “Send to mail-in database” as the journaling method, use this field to enter the name of the mail-in database to which the Router forwards messages to be journaled.
Click the down-arrow to select the name of the mail-in database from the Domino Directory.
Note You must create the mail-in database beforehand; Domino does not automatically create mail-in databases for journaling
If you need to stop duplicate mails as you are using a forwading rule (this is a legitimate duplicate as it is another document) you would need to create a rule that says “if the recipient is email@example.com” then do not Journal. This will stop your Duplicates.