An element with the same key already exists in the dictionary

I was testing an inventory system and ...

=-=-=

ArgumentException: An element with the same key already exists in the dictionary. System.Collections.Generic.Dictionary`2[System.String,System.String].Add (System.String key, System.String value) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:404) RPGItemDatabaseScript.ReadItemsFromDatabase () (at Assets/scripts/Items/RPGItemDatabaseScript.cs:40) RPGItemDatabaseScript.Awake () (at Assets/scripts/Items/RPGItemDatabaseScript.cs:17)

=-=-=

     using System.Collections;
     using System.Collections.Generic;
     using UnityEngine;
     using System.Xml;
     using System.IO;
     
     public class RPGItemDatabaseScript : MonoBehaviour {
     
         public TextAsset itemInventory;
         public static List<BaseItem> inventoryItem = new List<BaseItem> ();
     
         private List<Dictionary<string, string>> inventoryItemsDictionary = new List<Dictionary<string, string>>();
         private Dictionary<string, string> inventoryDictionary;
     
         void Awake(){
             
             ReadItemsFromDatabase ();
             for (int i = 0; i < inventoryItemsDictionary.Count; i++) {
     
                 inventoryItem.Add (new BaseItem (inventoryItemsDictionary [i]));
     
             }
     
         }
     
         public void ReadItemsFromDatabase(){
     
             XmlDocument xmlDocument = new XmlDocument ();
             xmlDocument.LoadXml (itemInventory.text);
             XmlNodeList itemList = xmlDocument.GetElementsByTagName ("Item");
     
             foreach (XmlNode itemInfo in itemList) {
     
                 XmlNodeList itemContent = itemInfo.ChildNodes;
                 inventoryDictionary = new Dictionary<string, string>();
     
     
                 foreach (XmlNode content in itemContent) {
     
                     inventoryDictionary.Add(content.Name.ToString(), content.InnerText);
                     inventoryItemsDictionary.Add (inventoryDictionary);
     
                 }
     
             }
         }
     }

=-=-=-=

Xml file

 <Item>
   <ItemName>TestItem</ItemName>
   <ItemID>1</ItemID>
   <ItemTypes>WEAPON</ItemTypes>
   <ItemName>TestItem2</ItemName>
   <ItemID>2</ItemID>
   <ItemTypes>WEAPON</ItemTypes>
 </Item>

=-=-=

I already researched in some places, but all I found was answers saying to remove certain specific code, and did not explain right what caused the error.