We were experiencing the following error 4 times per minute.

Scheduler Exception :

AssemblyVersion: -1
Method:
FileName:
FileLineNumber: -1
FileColumnNumber: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
AbsoluteURL:
AbsoluteURLReferrer:
ExceptionGUID:
DefaultDataProvider:
InnerException: Add failed. Duplicate key value supplied.
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.CoreScheduler.AddToScheduleInProgress(ScheduleHistoryItem objScheduleHistoryItem) at DotNetNuke.Services.Scheduling.DNNScheduling.CoreScheduler.WorkStarted(SchedulerClient& objSchedulerClient)
StackTrace:
Source:

Environment:

DotNetNuke Version: 3.1.1

Data Provider: SqlDataProvider

.NET Framework: 1.1.4322.2032

So, I went into Host > Schedule and started disabling each Scheduled Task until I found the culprit. This was pretty easy to figure out because the only non-core scheduled tasks that was new was the following:

UserPaymentTools.BuyNow.PurgeShoppingCart, UserPaymentTools.BuyNow

AND

UserPaymentTools.BuyNow.ServicePayments, UserPaymentTools.BuyNow

I tried every combination and each task created 2 scheduler exceptions per minute, despite having a frequency of 1 and 3 hours. I have disabled both task and no longer experience the error. I plan on following up with the developer of this module and will post my comments in a separate entry as this is the second major issue I have experienced from 2 modules I have purchased from this individual and did not receive a response from the first one...