I'm filing this on behalf of a developer on my team (Lukas), so I may not disambiguate all of these details...

We hit a race condition in subscribing to notifications in Carbon. I have not had time to figure out if the problem exists in later versions of ODL. The errant behavior is that a subscriber can end up with multiple subscriptions to the same thing, such that a single notification causes multiple callbacks.

The problem occurs when subscribing more than once successively in a short enough time window. 

POST:
http://controller-ip:8181/restconf/operations/sal-remote:create-notification-stream

{   "input":    

}

GET:
http://controller-ip:8181/restconf/streams/stream/create-notification-stream/bnc-notification:node-status-notification,bnc-notification:event-notification

The Carbon version of the code is here

POST:
https://github.com/opendaylight/netconf/blob/stable/carbon/restconf/sal-rest-connector/src/main/java/org/opendaylight/restconf/restful/services/impl/RestconfInvokeOperationsServiceImpl.java
GET:
https://github.com/opendaylight/netconf/blob/stable/carbon/restconf/sal-rest-connector/src/main/java/org/opendaylight/restconf/restful/services/impl/RestconfStreamsSubscriptionServiceImpl.java

The master branch version of the code is here...

POST:
https://github.com/opendaylight/netconf/blob/master/restconf/restconf-nb-rfc8040/src/main/java/org/opendaylight/restconf/nb/rfc8040/rests/services/impl/RestconfInvokeOperationsServiceImpl.java
GET:
https://github.com/opendaylight/netconf/blob/master/restconf/restconf-nb-rfc8040/src/main/java/org/opendaylight/restconf/nb/rfc8040/rests/services/impl/RestconfStreamsSubscriptionServiceImpl.java

Looking at the code, the culprit is likely in 

./restconf/sal-rest-connector/src/main/java/org/opendaylight/restconf/restful/utils/SubscribeToStreamUtil.java:notifYangStream
./restconf/sal-rest-connector/src/main/java/org/opendaylight/restconf/restful/utils/SubscribeToStreamUtil.java:registerToListenNotification
org.opendaylight.controller.md.sal.dom.broker.impl.DOMNotificationRouter.registerNotificationListener

probably do synchronized inside of this:

    private static void registerToListenNotification(final NotificationListenerAdapter listener,
            final NotificationServiceHandler notificationServiceHandler) {
        if (listener.isListening()) { --->>>condition through which pass both threads in same time
            return;                       --->>>synchronized to listener is probably ideal because it is same for both calls
        }

        final SchemaPath path = listener.getSchemaPath();
        final ListenerRegistration<DOMNotificationListener> registration =
                notificationServiceHandler.get().registerNotificationListener(listener, path);

        listener.setRegistration(registration);

    }

The master branch shows similar logic so will suffer the same behavior.

The problem likely exists in NETCONF, where there is duplicate code...

./restconf/restconf-nb-bierman02/src/main/java/org/opendaylight/netconf/sal/restconf/impl/BrokerFacade.java

There is a method with the same name and body registerToListenNotification