Drop duplicate mail - Postfix

Postfix mail server praised by all technician.
but it has a small problem of using "aliases" to define email group.
For example, when someone sends mail to "jameschen" who in the group "all",
"jameschen" will receive two the same mail form the server.
because the group title "all" is "jameschan"'s alias and "jameschan" is in group "all",
So that, Posrfix thinks that is two different username and will not drop the same mail,
Even the mail which sent to "all" will translate to "jameschan".
To solve this problem, we can use "procmail" to filter duplicate mail,
Touch a execute file name ".procmailrc" content as following:

:0 Whc: msgid.lock
formail -D 8192 msgid.cache
:0 a:
/dev/null

the confiig file of Postfix "main.cf" should be write a command line

"mailbox_command = /usr/bin/procmail"

Then copy to each user home directory,
The some mail will be drop.