Just getting this known issue again. This is quite weird because it's happening on a clean install of the latest Umbraco version(4.7.1.1).

I've created some doc types and added some content. Then I've added some values in the template, saved the template. Opened up mu content node in the tree, tried to save the node and then:

[ArgumentException: An item with the same key has already been added.]
   System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +52
   System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +9374523
   Examine.LuceneEngine.Providers.LuceneIndexer.GetDataToIndex(XElement node, String type) +548
   UmbracoExamine.UmbracoContentIndexer.GetDataToIndex(XElement node, String type) +207
   Examine.LuceneEngine.Providers.LuceneIndexer.AddNodesToIndex(IEnumerable`1 nodes, String type) +139
   UmbracoExamine.UmbracoContentIndexer.AddSingleNodeToIndex(XElement node, String type) +150
   Examine.LuceneEngine.Providers.LuceneIndexer.ReIndexNode(XElement node, String type) +87
   UmbracoExamine.BaseUmbracoIndexer.ReIndexNode(XElement node, String type) +56
   UmbracoExamine.UmbracoContentIndexer.ReIndexNode(XElement node, String type) +164
   Examine.ExamineManager._ReIndexNode(XElement node, String type, IEnumerable`1 providers) +81
   UmbracoExamine.UmbracoEventManager.IndexMedia(Media sender) +171
   UmbracoExamine.UmbracoEventManager.Document_AfterSave(Document sender, SaveEventArgs e) +322
   umbraco.cms.businesslogic.web.SaveEventHandler.Invoke(Document sender, SaveEventArgs e) +0
   umbraco.cms.businesslogic.web.Document.FireAfterSave(SaveEventArgs e) +27
   umbraco.cms.businesslogic.web.Document.Save() +180
   umbraco.cms.presentation.editContent.Save(Object sender, EventArgs e) +1931
   System.EventHandler.Invoke(Object sender, EventArgs e) +0
   umbraco.controls.ContentControl.saveClick(Object Sender, ImageClickEventArgs e) +319
   umbraco.controls.ContentControl.savePublish(Object Sender, ImageClickEventArgs e) +20
   System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +115
   System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +120
   System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5563

Because I could not get rid of this issue and I was started just with a new install, I dropped all tables and re-installed umbraco again. Unfortunately this issue was re-appearing.

I hope someone can help me get rid of this issue, I just don't know what to modify.

I've checked the index, removed the index, re-created the index because of the lucene error but that didn't help.