An error occured while attaching module (Dynamicweb.Frontend.Content)

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 Dynamicweb.Extensibility.AddInManager.GetAddInClasses(Type AddInBaseType)
   at Dynamicweb.ContentModule.GetContentModules()
   at Dynamicweb.ContentModule.CreateModuleInstance(String moduleName)
   at Dynamicweb.Frontend.Content.getModule(DataRow& ParagraphRow)