Good to refresh: log4j additivity against double logging

https://logging.apache.org/log4j/1.2/manual.html

The addAppender method adds an appender to a given logger. Each enabled logging request for a given logger will be forwarded to all the appenders in that logger as well as the appenders higher in the hierarchy. In other words, appenders are inherited additively from the logger hierarchy. For example, if a console appender is added to the root logger, then all enabled logging requests will at least print on the console. If in addition a file appender is added to a logger, say C, then enabled logging requests for C and C's children will print on a file and on the console. It is possible to override this default behavior so that appender accumulation is no longer additive by setting the additivity flag to false.

The rules governing appender additivity are summarized below.

Appender Additivity

    The output of a log statement of logger C will go to all the appenders in C and its ancestors. This is the meaning of the term "appender additivity".

    However, if an ancestor of logger C, say P, has the additivity flag set to false, then C's output will be directed to all the appenders in C and its ancestors upto and including P but not the appenders in any of the ancestors of P.

    Loggers have their additivity flag set to true by default.

in short,

<Loggername="com.foo.Bar"level="trace"additivity="false">

http://logging.apache.org/log4j/2.x/manual/configuration.html