If a remote interface is implemented by more than one class in a class hierarchy from which stubs/skeletons are generated, rmic will produce illegal java code. Here is an example:

interface Echo extends java.rmi.Remote { ... }
class EchoImpl extends java.rmi.server.UnicastRemoteObject implements Echo {...}
class SubEchoImpl extends EchoImpl implements Echo { ... }

rmic will generated illegal java code for SubEchoImpl. The resulting stub code will implement the interface Echo twice for the same class.