Please create a new file
data/conf/postfix/extra.cf for overrides or additional content to
Postfix will complain about duplicate values once after starting postfix-mailcow, this is intended.
Syslog-ng was configured to hide those warnings while Postfix is running, to not spam the log files with unnecessary information every time a service is used.
postfix-mailcow to apply your changes:
docker-compose restart postfix-mailcow