each appender thus referenced is added to the named logger. Note that unlike log4j, logback-classic does not close or remove any previously referenced appenders before adding the new appender reference.
In log4j version 1.2.16 , any previously referenced appenders are valid.
<logger name="classlogger" additivity="false">
<level value="INFO" />
<appender-ref ref="MYINFO_FILE" />
<appender-ref ref="DEBUG" />
</logger>
So, the manual is wrong.