Hi, when I use the MBean architecture with mx4j implementation to manage the 
logging parameters, I have an InstanceAlreadyExistsException. This happen when 
the org.apache.log4j.jmx.AppenderDynamicMBean try to register a layout 
(registerLayoutMBean method) because do not check if the MBean is already 
registered. Here there is the code that I use

MBeanServer server = MBeanServerFactory.createMBeanServer();
HierarchyDynamicMBean hdm = new HierarchyDynamicMBean();
...
LoggerRepository r = LogManager.getLoggerRepository();
Enumeration enum = r.getCurrentLoggers();
Logger logger = null;
while (enum.hasMoreElements()) {
  logger = (Logger) enum.nextElement();
  ObjectName dummy = new ObjectName("log4j", "logger", logger.getName());
  if (!server.isRegistered(dummy))
    hdm.addLoggerMBean(logger.getName());
}

and the log4j.properties

# Console is set to be a ConsoleAppender.
log4j.appender.Console=org.apache.log4j.ConsoleAppender
log4j.appender.Console.layout=org.apache.log4j.PatternLayout
log4j.appender.Console.layout.ConversionPattern=%d %-5p %c{1}.%M - %m%n

# FileConsole is set to be a FileAppender.
log4j.appender.FileConsole.File=\\wcubecoils.log
log4j.appender.FileConsole=org.apache.log4j.FileAppender
log4j.appender.FileConsole.layout=org.apache.log4j.PatternLayout
log4j.appender.FileConsole.layout.ConversionPattern=%d %-5p %c{1}.%M - %m%n

# the root logger
log4j.rootLogger=INFO, FileConsole

# wcube logging configuration...
log4j.logger.it.mbm.wcube.processmessage=TRACE, FileConsole
log4j.logger.it.mbm.wcube.mq.bo.action=TRACE, FileConsole