The 'Access-Control-Allow-Origin' header contains multiple values '*, *', but only one is allowed. Origin 'null' is therefore not allowed access.

Question

Hello. I added custom http response headers to my SP site web config file as follows: 
<httpProtocol>
      <customHeaders>
             <add name="Access-Control-Allow-Methods" value="POST,GET,OPTIONS" />
      <add name="Access-Control-Allow-Origin" value="*" />
      <add name="Access-Control-Allow-Headers" value="Content-Type,Authorization" />
      </customHeaders>
    </httpProtocol>
When I try to call any web service, i get these headers two times each: 
HTTP/1.1 200 OK
Cache-Control: private, max-age=0
Transfer-Encoding: chunked
Content-Type: application/atom+xml;type=entry;charset=utf-8
Expires: Sat, 01 Mar 2014 19:11:37 GMT
Last-Modified: Sun, 16 Mar 2014 19:11:37 GMT
ETag: "3"
X-SharePointHealthScore: 0
SPClientServiceRequestDuration: 20
SPRequestGuid: b4e77d9c-bfc3-a050-493a-ca5d251d1a72
request-id: b4e77d9c-bfc3-a050-493a-ca5d251d1a72
X-FRAME-OPTIONS: SAMEORIGIN
Persistent-Auth: true
Access-Control-Allow-Methods: POST,GET,OPTIONS
Access-Control-Allow-Origin: *
Access-Control-Allow-Headers: Content-Type,Authorization
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Access-Control-Allow-Methods: POST,GET,OPTIONS
Access-Control-Allow-Origin: *
Access-Control-Allow-Headers: Content-Type,Authorization
MicrosoftSharePointTeamServices: 15.0.0.4569
Date: Sun, 16 Mar 2014 19:11:37 GMT
and that gives me error from ajax: The 'Access-Control-Allow-Origin'
header contains multiple values '*, *', but only one is allowed. Origin 'null' is therefore not allowed access.
The 'Access-Control-Allow-Origin' header contains multiple values '*, *', but only one is allowed.
 Origin 'null' is therefore not allowed access.
Any idea???

Answer 1

Hi Ann,
Please check whether there are duplicate custom headers in your code.
Similar issue for your reference:
http://social.msdn.microsoft.com/Forums/office/en-US/b79b75f4-b46b-46ae-ae29-17a352b6b90b/custom-http-response-headers-for-sp-2013-shown-2-times?forum=sharepointdevelopment 
Regards,
Rebecca Tu
TechNet Community Support