I have a DNN 9.9.1 in which there are two portals.  One portal is working fine. For the second portal, going to edit mode results in this:

Server Error in '/' Application.

 An entry with the same key already exists.
  Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

  Exception Details: System.ArgumentException: An entry with the same key already exists.

 Source Error:

    [No relevant source lines]

  Source File:  c:\inetpub\vhosts\dev.hsc.com\tmp\root\d204065e\85bd8cfe\App_Web_vlvcbqh0.2.cs  Line:  0

 Stack Trace:

    [ArgumentException: An entry with the same key already exists.]
   System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +52
   System.Collections.Generic.SortedList`2.Add(TKey key, TValue value) +6893600
   DotNetNuke.Entities.Modules.DesktopModuleController.GetPortalDesktopModules(Int32 portalId) +196
   DotNetNuke.Web.Components.Controllers.ControlBarController.GetCategoryDesktopModules(Int32 portalId, String category, String searchTerm) +177
   Dnn.EditBar.UI.Items.AddModuleMenu.Visible() +80
   Dnn.EditBar.UI.Controllers.<>c.b__4_0(BaseMenuItem m) +12
   System.Linq.WhereListIterator`1.MoveNext() +73
   System.Linq.Buffer`1..ctor(IEnumerable`1 source) +192
   System.Linq.d__1.MoveNext() +175
   System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +387
   System.Linq.Enumerable.ToList(IEnumerable`1 source) +58
   Dnn.EditBar.UI.Controllers.EditBarController.GetMenuItems() +500
   Dnn.EditBar.UI.Controllers.ContentEditorManager.OnInit(EventArgs e) +257
   System.Web.UI.Control.InitRecursive(Control namingContainer) +139
   System.Web.UI.Control.AddedControl(Control control, Int32 index) +185
   System.Web.UI.ControlCollection.Add(Control child) +86
   Dnn.EditBar.UI.HttpModules.EditBarModule.OnSkinInit(Object sender, SkinEventArgs e) +327
   DotNetNuke.UI.Skins.Skin.InvokeSkinEvents(SkinEventType skinEventType) +285
   DotNetNuke.UI.Skins.Skin.OnInit(EventArgs e) +512
   System.Web.UI.Control.InitRecursive(Control namingContainer) +139
   System.Web.UI.Control.AddedControl(Control control, Int32 index) +185
   System.Web.UI.ControlCollection.Add(Control child) +86
   DotNetNuke.Framework.DefaultPage.OnInit(EventArgs e) +1584
   System.Web.UI.Control.InitRecursive(Control namingContainer) +139
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +408

[PageLoadException: An entry with the same key already exists.]
   DotNetNuke.Services.Exceptions.Exceptions.ProcessPageLoadException(Exception exc, String URL) +468
   DotNetNuke.Framework.PageBase.OnError(EventArgs e) +743
   System.Web.UI.Page.HandleError(Exception e) +84
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6560
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +245
   System.Web.UI.Page.ProcessRequest() +72
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +22
   System.Web.UI.Page.ProcessRequest(HttpContext context) +58
   ASP.default_aspx.ProcessRequest(HttpContext context) in c:\inetpub\vhosts\dev.hsc.com\tmp\root\d204065e\85bd8cfe\App_Web_vlvcbqh0.2.cs:0
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +188
   System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +48
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +71

---------------------------

I can clear the error by opening the browser's developer tools, go to Storage and remove LastPageId.  Then I can get back to the page, but I'm not in Edit mode.

If I create a third portal with the Blank template, the same error occurs.  I cannot get into Edit mode.

I'm looking for suggestgions to fix this ...