title: how to add duplicate keys to dictionary<string,string>
body: <p>how to add duplicate keys to dictionary</p>
  
  <p>i.e i have already added the key,value pair as rollno,1
  but i need to add the same parameter to the dictionary,but it is not allowing to add. how to add duplicated keys/repeated key in dictionary
  .</p>
  
  <p>or any other choice..
  any idea???</p>