JBOss 4.2 version we are using and since 8 months I'm using this. Today morning I created a mail-service .xml (save as with different name form already existed mail-service.xml), and I got the duplicate name "jboss:service=Mail", then immediately I removed the new file then onwards I struck with this exception.
Perhaps it is picking up a different JAR file for naming. Try adding "-verbose:class" to the JAVA_OPTS in the run script (run.bat for Windows, run.conf for Linux). This JVM option causes the JVM to print each class loaded and identify each JAR file. Look for JAR files not in the JBoss AS directories. Especially look for the loafing of the class org.jnp.server.NamingBeanImpl, which implements the jboss:service=NamingBeanImpl MBean.