Idictionary if statements C++

I'm just asking in general. I have Idictionary with multiple values added to make a specific key from a normal dictionary variable. I am getting an error where it is saying I am adding something with the same specific key which I understand why. I am just confused on how to make an if-statement for the idictionary variable so like:

if idictionary cannot add key because it contains a key that is the same
{
    ignore adding value. 
}

How may I implement this? Thanks!