grok pattern to filter postfix gets duplicated fields (crosspost from /r/logstash)

When I try to create a custom grok pattern to filter postfix component (qmgr, anvil,...), some fields added unexpectedly.

My logstash configuration file: http://pastebin.com/PhYDELss

My custom postfix grok pattern: http://pastebin.com/Nu1iExib

When running logstash, some fields duplicated: (component, hostname, pid, timestamp) : http://i.imgur.com/b3oTyNE.png

Could someone explain why this happened? And how can I remove the duplicated field?

Thanks