I have an scheduler that sends out emails if some conditions are met.

My problem are that sometimes these emails are sent out twice. 

And in my EvenLog this scheduler exception occurs when the email is sent twice: (I also have an Application started right before the error)

DefaultDataProvider:
ExceptionGUID:
InnerException: Add failed. Duplicate key value supplied.
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: System.ArgumentException: Add failed. Duplicate key value supplied. at Microsoft.VisualBasic.Collection.Add(Object Item, String Key, Object Before, Object After) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.AddToScheduleInProgress(ScheduleHistoryItem objScheduleHistoryItem) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.WorkStarted(SchedulerClient& objSchedulerClient)
 

I have tried to set the SearchEngineScheduler from 30 min to 4 hrs.

I also noticed that the SendLogNotification was run 1 sek before the error.

By the way i run DNN 4.5.5

Any suggestions on how to solve my problems?

Planning to upgrade to 4.9 in the near future, will that fix my problem?