How to add duplicate keys into the Dictionary

Question

I have some lines from text files that i want to add into the Dictionary.I am using Dictionary for the first time.While adding up starting lines it was Ok but suddenly i got err

Answer 1

List< KeyValuePair < string, string>> listKeyValPair= new List< KeyValuePair< string, string>>();
KeyValuePair< string, string> keyValue= new KeyValuePair< string, string>("KEY1", "VALUE1");
listKeyValPair.Add(keyValue);