Today we had the same error:

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 System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value) at DotNetNuke.Entities.Modules.ModuleInfo.get_LocalizedModules()


This SQL helped us to find TabModules causing that error:

SELECT dbo.Tabs.TabID, dbo.Tabs.TabName, dbo.Tabs.TabPath, DuplicateTabModules.CultureCode, TabModules_1.ModuleID, TabModules_1.IsDeleted,
DuplicateTabModules.Count, dbo.ModuleDefinitions.FriendlyName, TabModules_1.PaneName, dbo.Modules.AllTabs, TabModules_1.ModuleTitle,
TabModules_DefaultLanguage.ModuleTitle AS [Module Title (Default Language)], TabModules_DefaultLanguage.TabID AS [TabID (Default Language)],
Tabs_DefaultLanguage.TabName AS [TabName (Default Language)], Tabs_DefaultLanguage.TabPath AS [TabPath (Default Language)],
DuplicateTabModules.DefaultLanguageGuid, dbo.Tabs.PortalID
FROM (SELECT CultureCode, DefaultLanguageGuid, COUNT(*) AS Count
FROM dbo.TabModules
GROUP BY CultureCode, DefaultLanguageGuid
HAVING (COUNT(*) > 1) AND (DefaultLanguageGuid IS NOT NULL)) AS DuplicateTabModules INNER JOIN
dbo.TabModules AS TabModules_1 ON DuplicateTabModules.DefaultLanguageGuid = TabModules_1.DefaultLanguageGuid AND
DuplicateTabModules.CultureCode = TabModules_1.CultureCode INNER JOIN
dbo.Tabs ON TabModules_1.TabID = dbo.Tabs.TabID INNER JOIN
dbo.Modules ON TabModules_1.ModuleID = dbo.Modules.ModuleID INNER JOIN
dbo.ModuleDefinitions ON dbo.Modules.ModuleDefID = dbo.ModuleDefinitions.ModuleDefID INNER JOIN
dbo.TabModules AS TabModules_DefaultLanguage ON TabModules_1.DefaultLanguageGuid = TabModules_DefaultLanguage.UniqueId INNER JOIN
dbo.Tabs AS Tabs_DefaultLanguage ON TabModules_DefaultLanguage.TabID = Tabs_DefaultLanguage.TabID

Same problem here, we have a website with 4 languages. When I try to add a page I get the same error and the page is only added in 2 languages.

Used the SQL select query above as result I get the banner module which is placed on all pages (Display module on all pages). Looks like the problem is related to modules that are copied or displayed on all pages. Deleting this module and cleaning the recycle bin solved the problem for now, but we really hope there wil be a fix for this localization bug!!!!