received two identical emails when setting email forward

[tiger2000]

Dear All,

problem:
user1@aa.example.com sets email forward to user2@aa.example.com,
but user2@aa.example.com will receive two identical emails when sending email to user1@aa.example.com.
Except the email forwarding, everyone receives inbound/outbound emails in correct behavior.

environment:
example.com is the company domain, but we separate into several sub-domain according to the regions, so that it would look like:

@aa.example.com
@bb.example.com

in public DNS configuration, all DNS MX are pointed to the company anti-spam gateway, and the anti-spam gateway directs emails to zimbra mail server(which's in local private 192.168.xx.xx IP).

the zimbra mail server runs a local DNS server which is only for itself. the IP address is set to 192.168.xx.xx, and its /etc/hosts and DNS FQDN is set to something like: mail.example.com.


From the maillog file, we found it did sends two copies to user2@aa.example.com, please check below:

Nov  9 16:55:53 mail postfix/smtpd[29107]: connect from spam.example.com[192.168.12.14]
Nov  9 16:55:53 mail postfix/smtpd[29107]: D724DC08001: client=spam.example.com[192.168.12.14]
Nov  9 16:55:53 mail postfix/cleanup[29110]: D724DC08001: message-id=<21283398.59931289292951567.JavaMail.root@xxx.xxx.xxx>


Nov  9 16:55:53 mail postfix/qmgr[27895]: D724DC08001: from=<xxx@xxx.xxx.xxx>, size=2282, nrcpt=2 (queue active)
Nov  9 16:55:53 mail postfix/smtpd[29107]: disconnect from spam.example.com[192.168.12.14]
Nov  9 16:55:53 mail postfix/pickup[27896]: F0238C08009: uid=500 from=<xxx@xxx.xxx.xxx>
Nov  9 16:55:53 mail postfix/pipe[29111]: D724DC08001: to=<user1@aa.example.com>, relay=filter, delay=0.1, delays=0.05/0/0/0.05, dsn=2.0.0, status=sent (delivered via filter service)
Nov  9 16:55:53 mail postfix/pipe[29111]: D724DC08001: to=<user2@aa.example.com>, orig_to=<user1@aa.example.com>, relay=filter, delay=0.1, delays=0.05/0/0/0.05, dsn=2.0.0, status=sent (delivered via filter service)
Nov  9 16:55:53 mail postfix/qmgr[27895]: D724DC08001: removed
Nov  9 16:55:53 mail postfix/cleanup[29110]: F0238C08009: message-id=<21283398.59931289292951567.JavaMail.root@xxx.xxx.xxx>
Nov  9 16:55:54 mail postfix/qmgr[27895]: F0238C08009: from=<xxx@xxx.xxx.xxx>, size=2431, nrcpt=3 (queue active)
Nov  9 16:55:54 mail postfix/lmtp[28592]: F0238C08009: to=<user1@aa.example.com>, relay=mail.example.com[192.168.12.15]:7025, delay=0.28, delays=0.07/0/0/0.21, dsn=2.1.5, status=sent (250 2.1.5 Delivery OK)
Nov  9 16:55:54 mail postfix/lmtp[28592]: F0238C08009: to=<user2@aa.example.com>, orig_to=<user1@aa.example.com>, relay=mail.example.com[192.168.12.15]:7025, delay=0.28, delays=0.07/0/0/0.21, dsn=2.1.5, status=sent (250 2.1.5 Delivery OK)
Nov  9 16:55:54 mail postfix/lmtp[28592]: F0238C08009: to=<user2@aa.example.com>, relay=mail.example.com[192.168.12.15]:7025, delay=0.28, delays=0.07/0/0/0.21, dsn=2.1.5, status=sent (250 2.1.5 Delivery OK)
Nov  9 16:55:54 mail postfix/qmgr[27895]: F0238C08009: removed

any advice on this situation??
Thanks.