I am having an intermittent issue that is appearing on one server only, and is causing all MVC pages to fail to load with the error "An item with the same key has already been added." Restarting the application pool fixes the issue, but until then, loading any mvc page throws the following exception: Event code: 3005 Event message: An unhandled exception has occurred. Event time: 10/11/2012 08:09:24 Event time (UTC): 10/11/2012 08:09:24 Event ID: d76264aedc4241d4bce9247692510466 Event sequence: 6407 Event occurrence: 30 Event detail code: 0 Application information: Application domain: /LM/W3SVC/21/ROOT-2-129969647741292058 Trust level: Full Application Virtual Path: / Application Path: d:\websites\SiteAndAppPoolName\ Machine name: UKSERVER Process information: Process ID: 6156 Process name: w3wp.exe Account name: IIS APPPOOL\SiteAndAppPoolName Exception information: Exception type: ArgumentException Exception message: An item with the same key has already been added. Server stack trace: at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer) at System.Web.WebPages.Scope.WebConfigScopeDictionary.<>c__DisplayClass4.<.ctor>b__0() at System.Lazy`1.CreateValue() Exception rethrown at [0]: at System.Lazy`1.get_Value() at System.Web.WebPages.Scope.WebConfigScopeDictionary.TryGetValue(Object key, Object& value) at System.Web.Mvc.ViewContext.ScopeGet[TValue](IDictionary`2 scope, String name, TValue defaultValue) at System.Web.Mvc.ViewContext.ScopeCache.Get(IDictionary`2 scope, HttpContextBase httpContext) at System.Web.Mvc.ViewContext.GetClientValidationEnabled(IDictionary`2 scope, HttpContextBase httpContext) at System.Web.Mvc.Html.FormExtensions.FormHelper(HtmlHelper htmlHelper, String formAction, FormMethod method, IDictionary`2 htmlAttributes) at System.Web.Mvc.Html.FormExtensions.BeginForm(HtmlHelper htmlHelper, String actionName, String controllerName) at ASP._Page_Views_Dashboard_Functions_BookingQuickLookup_cshtml.Execute() in d:\Websites\SiteAndAppPoolName\Views\Dashboard\Functions\BookingQuickLookup.cshtml:line 3 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model, ViewDataDictionary viewData) at ASP._Page_Views_Dashboard_Functions_cshtml.Execute() in d:\Websites\SiteAndAppPoolName\Views\Dashboard\Functions.cshtml:line 5 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.Mvc.Html.RenderPartialExtensions.RenderPartial(HtmlHelper htmlHelper, String partialViewName, Object model) at ASP._Page_Views_Dashboard_Index_cshtml.Execute() in d:\Websites\SiteAndAppPoolName\Views\Dashboard\Index.cshtml:line 9 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) at System.Web.Mvc.Controller.ExecuteCore() at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5() at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0() at System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d() at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Request information: Request URL: http://SiteAndAppPoolName.spawtz.com/Dashboard Request path: /Dashboard User host address: 86.164.135.41 User: Is authenticated: False Authentication Type: Thread account name: IIS APPPOOL\SiteAndAppPoolName Thread information: Thread ID: 17 Thread account name: IIS APPPOOL\SiteAndAppPoolName Is impersonating: False Stack trace: at System.Lazy`1.get_Value() at System.Web.WebPages.Scope.WebConfigScopeDictionary.TryGetValue(Object key, Object& value) at System.Web.Mvc.ViewContext.ScopeGet[TValue](IDictionary`2 scope, String name, TValue defaultValue) at System.Web.Mvc.ViewContext.ScopeCache.Get(IDictionary`2 scope, HttpContextBase httpContext) at System.Web.Mvc.ViewContext.GetClientValidationEnabled(IDictionary`2 scope, HttpContextBase httpContext) at System.Web.Mvc.Html.FormExtensions.FormHelper(HtmlHelper htmlHelper, String formAction, FormMethod method, IDictionary`2 htmlAttributes) at System.Web.Mvc.Html.FormExtensions.BeginForm(HtmlHelper htmlHelper, String actionName, String controllerName) at ASP._Page_Views_Dashboard_Functions_BookingQuickLookup_cshtml.Execute() in d:\Websites\SiteAndAppPoolName\Views\Dashboard\Functions\BookingQuickLookup.cshtml:line 3 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.Mvc.Html.PartialExtensions.Partial(HtmlHelper htmlHelper, String partialViewName, Object model, ViewDataDictionary viewData) at ASP._Page_Views_Dashboard_Functions_cshtml.Execute() in d:\Websites\SiteAndAppPoolName\Views\Dashboard\Functions.cshtml:line 5 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.Mvc.Html.RenderPartialExtensions.RenderPartial(HtmlHelper htmlHelper, String partialViewName, Object model) at ASP._Page_Views_Dashboard_Index_cshtml.Execute() in d:\Websites\SiteAndAppPoolName\Views\Dashboard\Index.cshtml:line 9 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) at System.Web.Mvc.Controller.ExecuteCore() at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5() at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0() at System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d() at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Custom event details: As mentioned, it's every MVC action that throws this error until the app pool is restarted, and the error seems to be occurring in System.Web.WebPages.Scope.WebConfigScopeDictionary.TryGetValue(Object key, Object& value) Has anyone seen this issue before? It's only happening on this server, on any of the app pools on the server (not confined to this one) and an app pool restart sorts it. Any help much appreciated. Cheers, Matthew