[logback-user] How to get a unique content rollingFileAppender?

I want such a rollingFileAppneder:Before appending new line to the current log file, the logger will check: if there is any same line in the current log file, it will not write the same content to the file. I can not find such a file appender, so I want to customize it: Firstly, I want to create a class extends the RollingFileAppneder, but I find that the field which point to current file in RollingFileAppneder is "friendly", I can not get the field in my own class. There is an stupid option: I copy all the codes in the RollingFileAppneder, and changed some logic in the append part. Is there a elgant way to get such a unique content rollingFileAppender?