An item with the same key has already been added

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An item with the same key has already been added.
[ArgumentException: An item with the same key has already been added.]
   System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +48
   System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +2668392
   EPiServer.Security.AccessControlList.Add(AccessControlEntry ace) +80
   EPiServer.DataAccess.PageListDB.AclLoader(IDataReader reader, PageDataFacade page) +172
   EPiServer.DataAccess.PageListDB.PageRecordEnumerator(IDataReader reader, IEnumerator pagesEnum, PageRecordDelegate loadMethod) +179
   EPiServer.DataAccess.PageListDB.ReadPageList(AbstractCommand cmd) +685
   EPiServer.DataAccess.PageListDB.LoadSpecificPages(Int32[] pageLinks) +421
   EPiServer.LocalPageStore.ReadMissingPages(PageReferenceCollection missingPages) +234
   EPiServer.LocalPageStore.GetScatteredPages(PageReferenceCollection pageRefs) +444
   EPiServer.LocalPageStore.GetChildren(PageReference pageLink, ILanguageSelector selector, Int32 startIndex, Int32 maxRows) +160
   EPiServer.DataFactory.GetChildren(PageReference pageLink, ILanguageSelector selector, Int32 startIndex, Int32 maxRows) +277
   EPiServer.DataFactory.GetChildren(PageReference pageLink) +77
   Foundation.Library.UI.WebControls.PageTree.CreateChildControlsRecursive(PageData page, Control container, Int32 level) in C:\Documents and Settings\andreas.norman\My Documents\Visual Studio 2005\Projects\Clients\TyrensWeb\Foundation\Library\UI\WebControls\PageTree.cs:309
   Foundation.Library.UI.WebControls.PageTree.CreateControlHierarchy() in C:\Documents and Settings\andreas.norman\My Documents\Visual Studio 2005\Projects\Clients\TyrensWeb\Foundation\Library\UI\WebControls\PageTree.cs:303
   Foundation.Library.UI.WebControls.PageTree.DataBindChildren() in C:\Documents and Settings\andreas.norman\My Documents\Visual Studio 2005\Projects\Clients\TyrensWeb\Foundation\Library\UI\WebControls\PageTree.cs:273
   System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) +216
   System.Web.UI.Control.DataBind() +12
   Foundation.Library.UI.WebControls.PageTree.CreateChildControls() in C:\Documents and Settings\andreas.norman\My Documents\Visual Studio 2005\Projects\Clients\TyrensWeb\Foundation\Library\UI\WebControls\PageTree.cs:266
   System.Web.UI.Control.EnsureChildControls() +87
   System.Web.UI.Control.PreRenderRecursiveInternal() +50
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2041