FYI - I was able to fix this issue by setting my caching defaultProvider to "BroadcastPollingCachingProvider"

This is the recommended setting for the Web Farm environment (alternative config) I am using.

Advice:  Don't think you remember all the settings for Web Farm support just because you've done it a hundred times!   :-)

Hi, I am still encountering this problem. Has anyone a solution yet?

AssemblyVersion: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
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)
Source:
Server Naam: XX

Assume that this process is started more than once...Can it be that a singleton or monitor must be used to avoid the creation of double keys? I did not study the sourcecode yet. But maybe this is the problem?

Can it be that due to the web application setup project way two "core modules" can exist? For instance, I have used the source code for development and use it as well in production. I have seen the compiler creates a dll of about 3MB of the source project. Maybe this is the problem?