Error debugging on iOS from Visual Studio

I am getting this error when debugging on an iPhone simulator in Visual Studio running Win8.1x64 in a Parallels VM:

            System.ArgumentException: An item with the same key has already been added.
               at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
               at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
               at Xamarin.VisualStudio.Debugger.Process.OnThreadStart(Object sender, TargetEventArgs args)
               at Mono.Debugging.Client.DebuggerSession.OnTargetEvent(TargetEventArgs args)
               at Mono.Debugging.Soft.SoftDebuggerSession.HandleThreadStartEvents(ThreadStartEvent[] events)
               at Mono.Debugging.Soft.SoftDebuggerSession.HandleEventSet(EventSet es)
               at Mono.Debugging.Soft.SoftDebuggerSession.EventHandler()

VS2013 pops up a modal dialog that requires I acknowledge it before I have to stop my debugger session.

Not sure what's happening here, but there are many threads being started and stopped based on the Debug output window. I haven't tried debugging the project from Xamarin Studio on my Mac.

Thanks,
Matthew