Allow duplicate event sinks to be registered to allow retries of component activation.

Change-Id: I892f0efc45239c7399224c3754b640256a9205e9
(cherry picked from commit a255470be96ef72f5a72e1bd80d9744bf5f4cea0)
diff --git a/core/api/src/main/java/org/onosproject/event/DefaultEventSinkRegistry.java b/core/api/src/main/java/org/onosproject/event/DefaultEventSinkRegistry.java
index 7e2f551..d934a3b 100644
--- a/core/api/src/main/java/org/onosproject/event/DefaultEventSinkRegistry.java
+++ b/core/api/src/main/java/org/onosproject/event/DefaultEventSinkRegistry.java
@@ -36,8 +36,6 @@
     public <E extends Event> void addSink(Class<E> eventClass, EventSink<E> sink) {
         checkNotNull(eventClass, "Event class cannot be null");
         checkNotNull(sink, "Event sink cannot be null");
-        checkArgument(!sinks.containsKey(eventClass),
-                      "Event sink already registered for %s", eventClass.getName());
         sinks.put(eventClass, sink);
     }