If you really want to make sure your code is reliable put some
checking in place before you add the entry.

StringDictionary stringDic = new StringDictionary();
string stringKey = "FirstOne";
stringDic.Add(stringKey,"One");

//comment out below and an error occurs
//stringDic.Add(stringKey,"Two");

//in your code before you add your entry check to see if it exists
if(stringDic.ContainsKey(stringKey) = false)
{
    stringDic.Add(stringKey,"Two");
}

- Adam