using System;

using System.Collections;

using System.Collections.Generic;

​

public class Program

{

     public static void Main()

     {

          var dic = new DuplicateKeyDictionary();

          dic.Add("Key1", "test");

          dic.Add("Key2", "test2");

          dic.Add("Key3", "test3");

          dic.Add("Key1", "test4");

​

          foreach(var item in dic)

          {

               Console.WriteLine($"{item}");

          }

     }

}

​

​

public class DuplicateKeyDictionary : IEnumerable

{

     private List<object> keys = new List<object>();

     private List<object> values = new List<object>();

​

     public void Add(object key, object value)

     {

          keys.Add(key);

          values.Add(value);

     }

​

     public IEnumerator GetEnumerator()

     {

          for (int i = 0; i < keys.Count; i++)

          {

               yield return new KeyValuePair<object, object>(keys[i], values[i]);

          }

     }

​

     public object this[object key]

     {

          get

          {

               for (int i = 0; i < keys.Count; i++)

               {

                    if (keys[i].Equals(key))

                    {

                         return values[i];

                    }

               }

               return null;

          }

     }

}