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<@UDLCP0K9T> are you able to send the headers the server is sending back? My first guess is that you have the filter being applied twice, hence duplicating the header, but it's hard to tell without looking at how things are setup on you side.

thanks Ivan for replying. I will share the information here once I have a chance.

Hi <@U3JDL750W>, I'm working with Xuemin on this. These are the details:

The browser "error" from this request:

```
Access to fetch at '<http://localhost:8001/api/v2/permissions>' from origin '<https://www.dev.flexi.uk>' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header contains multiple values '<https://www.google.com>, <https://www.bbc.co.uk>', but only one is allowed. Have the server send the header with a valid value, or, if an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
```

The response headers from this request:

```
access-control-allow-headers: content-type
access-control-allow-methods: POST, OPTIONS
access-control-allow-origin: <https://www.google.com>, <https://www.bbc.co.uk>
```

I think the specification for CORS says `"Access-Control-Allow-Origin" ":" origin-list-or-null | "*"`, which seems to match the http4k implementation

<https://www.w3.org/TR/cors/#access-control-allow-origin-response-header>

However it goes on to say `In practice the origin-list-or-null production is more constrained. Rather than allowing a space-separated list of origins, it is either a single origin or the string "null".`

Yeap. It seems like the filter needs to take into account the `Origin` header coming from the client and return a single allowed origin, if that's in the list.

May I suggest copying the current filter to your codebase and modifying it to behave in such way? If that works for you we're happy to incorporate the fix back in http4k.

Perfect. Yes we assumed the approach to resolve will be something like this :slightly_smiling_face: Great, we'll get it working locally, and then send you a pull request or whatever works best for you guys :thumbsup:

Excellent. Let me know how things go, and if you want to submit a PR, even better :slightly_smiling_face:

Hi <@U3JDL750W> we've created a pull request, which you can find here - <https://github.com/http4k/http4k/pull/199> - Happy to discuss, just let us know :thumbsup:

That looks ok to me. Did it work fine for your specific use case?

Yes we've written some tests that cover our use case, and we're pretty happy with them :slightly_smiling_face:

Cool. Merged then. Will be available on the next version. Are you in a hurry to remove your version from the codebase?

Great, thanks a lot :thumbsup: We're not in a hurry really. Do you know when the next version might be release though?

We'll release 3.103.2 with your fix today

releasing now. should be in jcenter inside the next 20 minutes or so.