X-Frame-Options: Nginx includes header twice

[Some Developer]

I'm running Nginx 1.4.4 on Ubuntu 12.04 and have added the 
X-Frame-Options header for one of my sites but in testing it appears 
that Nginx includes this itself in addition to user configured headers. 
Basically I want X-Frame-Options to be DENY but when I set that header 
Nginx also sends an X-Frame-Options SAMEORIGIN header so that there are 
two X-Frame-Options headers in every request.

Is there some way to disable the extra header? I can't find anything in 
my configuration that would add the second header.
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