C# - Check If HttpContext.Current.Response Have An Existing Header?

I was working on a scenario where I have to add header to the Response Header.

I added a Response Header like this:- HttpContext.Current.Response.AddHeader("key1", "value1")

After some functional flow I required to add value to same "key1": HttpContext.Current.Reponse.AddHeader("key1, "value2")

thus value in Response Headers look like this:- key1 : value1, value2

I want to avoid the value2 addition to the same keyword key1.

I tried:-

If HttpContext.Current.Response.Headers("key1") Then
HttpContext.Current.Reponse.AddHeader("key1, "value2")
EndIf

But I got the error: This operation requires IIS integrated pipeline mode at HttpContext.Current.Response.Headers("key1")

I have IIS6 running. This [URL] says that it doestn't works if its not IIS7