Creating mail copies for backup

[Marc Mc Guinness]

Hello,

I don't know who I could ask. Probably you could help me...
I'm using sendmail 8.12.6-6 on Debian 3.0 and want to create a copy
of every email delivered to the user mailboxes under /var/mail/.
This copy shall be saved in user mailboxes under /var/backup/mail/.

How can I tell sendmail to write it's mails to two directories?

Best regards,

Marc Mc Guinness