append value to a dictionary

Hi! Is there a way to append a value to a distionary that already has a value in it.
Example
Dicyionary<string,string> dc = new Dictionary<string,string>();
dc.Add("key1", "value1");
Now I have a loop and I am adding key/value pairs to this dictioanry but I have some keys that are identical with different values
so, inside the loop:
if(dc.containkey(key1))
{
//example append value2 to key1
}
My goald is to end up with dictioanry of key, key1 and value of value1 + value2. Thanks for any help
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