All notifications will be saved to a table to be shared by all clients. When creating the table, we need to do two things:
1) adding a listener to an MBeanSever, it will receive a notification when a new mbean is registered to the MBeanServer, then we can add a listener to receive notifications from the mbean;
2) getting a set of mbeans registered already to add our listener to receive notifications of the mbeans.

Today's implementation will add two times its listener to an mbean if the mbean is registered before operation 2 (getting a set of registered mbeans) but its registry notification arrives after operation 2 is finished.

This problem will not happen often but better to fix it in a future JDK version.