Add warning in server logs when data event is ignored as a duplicate

Under certain rare conditions, a client may send or resend a data event with an eventId that causes the server to interpret it as a duplicate event and discard it.

It is currently impossible to trace when this happens without extra logging added.

From Barry:
No, if the server thinks it has seen the event, it silently eats it. See DistributedEventTracker.hasSeenEvent. It has a trace log message but thats it.

The log message that tracks this behavior in the server needs to be added permanently.