Hello! 

We are receiving this kind of error now and then but couldn't find a way to reproduce it. Is it a known issue that may be fixed in the next version? AddOns.Blocks package is up to date.

System.ArgumentException: Item has already been added. Key in dictionary: 'OptimizingBlockSession' Key being added: 'OptimizingBlockSession'
at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)
at System.Collections.Specialized.NameObjectCollectionBase.BaseAdd(String name, Object value)
at System.Web.SessionState.SessionStateItemCollection.set_Item(String name, Object value)
at EPiServer.Cms.AddOns.Blocks.Web.Sessions.OptimizingBlockSession.GetSession(String sessionName)
at EPiServer.Cms.AddOns.Blocks.InitializableModule.CheckGoal(Object sender, ValidateRequestAccessEventArgs validateRequestAccessEventArgs)
at EPiServer.SiteBase.OnValidateRequestAccess(ValidateRequestAccessEventArgs e)
at EPiServer.Global.Global_PreRequestHandlerExecute(Object sender, EventArgs e)
at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)