D_REJECT in custom module

[Aleksandar Lazic]

Dear Reader,

I use the Amavis::Custom possibility to check if a message id will be
deliverd a second time to the same recipient.

Similar like http://www.courier-mta.org/reformail.html

###
Detecting duplicate messages

The -D option implements a simple way to delete duplicate messages in
incoming mail. filename is a file that will be approximately 'len' bytes
long. This file will be used by reformail to save message IDs seen in
recent mail. reformail reads the message on standard input. If the
message has a Message-ID: header that's already in the cache file,
reformail terminates with the exit code set to 0. Otherwise, reformail
terminates with the exit code set to 1.
###

I was able to save the message-id and the recipient in a DB
(TokyoCabinet,http://fallabs.com/tokyocabinet/), but my current problem
is that I don't know how I can tell amavis that he don't need to deliver
the mail when the message-id is the same to this recipient?

Please can anybody help me and point me to the right direction.

Thank you.

BR
Aleks