two copies of every email when using always_bcc

[Stefan Guenther]

Hello,

we have set "always_bcc = postmappe@localhost" to get a copy of every 
incoming and outgoing email.

The problem now is, that we do not only get one copy of every incoming 
or outgoing email, but TWO.

There reason for this seems to be the content filter:

content_filter = smtp-amavis:[127.0.0.1]:10024

When I remove this line, I only get one copy.

Can anyone explain why using a content filter creates two copies and is 
there a way to avoid the second copy?

Thanks for any hints and suggestions,

Stefan