Anyone can give me a hint or advise how can I duplicate all emails coming on a user to a have a copy to another folder inside his Maildir directory.

Example.

There's a directory called backup inside the Maildir other than new, cur and tmp. Of course if someone email to a user, the email will go to new directory under Maildir. I want also to have a copy of that every new email to the backup folder inside the Maildir. I believe I can do this in .qmail inside the user's home directory. I just don't know where to start. Can anyone give me a url or suggestion how to do this. Can I create a simple bash script to put on .qmail to make this thing run?