[logback-user] The MDCPropertyMap has been already set for this event Exception

[Chen Jian]

Hi - 

I have wrote an appender which use `setMDCPropertyMap` to inject some fields to LoggingEvent. 
It’s fail to working with other build-in appender (e.g. ch.qos.logback.core.FileAppender)
Exception:
java.lang.IllegalStateException: The MDCPropertyMap has been already set for this event.

Regards,
Jian