Problem with rule in mailflow and smarthost. Duplicates

Hi,

We have a mailcontact X@X.mail.local.

We've create a Mailflow rule that says that all mails with subject "Y"  should be BCC'd to X.mail.local.

Then we've create a sendconnector, so that mail for X.mail.local should be forward to smarthost X.mail.local. A local smtp-server is waiting on X.mail.local.

This setup works, however if somebody sends a mail with subject Y with multiple To: or BCC: the email gets send X times to smarthost X.mail.local.

However.. this depends on from where you are sending.

If you send an email from Gmail to two contacts (bcc or to) the email only arrives 1 time at X.mail.local. If we send with another external ISP the mail comes in 2 times at X.mail.local (If its to 3 persons, 3 times)

Has somebody experienced this before?