Add note about duplicate PeriodicTimer messages when actors are busy (

#7396)

src/core/Akka/Actor/Scheduler/TimerScheduler.cs

@@ -293,6 +293,9 @@ public object InterceptTimerMsg(ILoggingAdapter log, ITimerMsg timerMsg)
                 return null; // message should be ignored
             }
 
+            // N.B. - repeating timers never change their generation, so this check always passes.
+            // This means that, in theory, a repeating timer can queue up the same message many times
+            // in the actor's mailbox (i.e. when actor is busy) and there's no means of de-duplicating it.
             if (timerMsg.Generation == timer.Generation)
             {
                 // valid timer