var queryString = QueryHelpers.ParseQuery("?param1=value");
// queryString.GetType() --> typeof(Dictionary<String,StringValues>)
// Adding a parameter
queryString.Add("param2", "my value"); // Easy so far.
// Adding a second parameter with the same name
queryString.Add("param2", "my other value"); // ArgumentException!
// --> An item with the same key has already been added