I completely agree with Olly. If a module would add the header, IIS would remove it (because of that remove line), and when two headers are delivered, the second is added after the whole process most propably.
In this case, there is nothing you can do in IIS. But you can test it, if you browse the website http://healthtest.co.santa-cruz.ca.us/ on the server where it is installed. Add something like
to your hosts-file (normally under C:\windows\system32\drivers\etc, you may edit this with any text editor) to avoid a DNS request.
Then check the headers you get there (and remove the line above from the hosts file after the test).