This would solve...

As outlined at WHATWG/fetch there is often the need in a server environment to access cookie headers.

For example editing Cookie headers:

const h = new Headers;
h.append("Set-Cookie", "a=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT");
h.append("Set-Cookie", "b=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT");

h.get("Set-Cookie")
// a=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT, b=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT

Splitting the combined header value by , will give an invalid result. Instead getAll could be used to retrieve the individual headers.

The implementation should look like...

There are two solutions currently proposed:

1. Node-fetch provides a non-standard Headers.prototype.raw method that returns entries for each (including duplicate) header.
2. Re-introduce Headers.prototype.getAll to return multiple header values.

I have also considered...

1. Not use undici primitives within NodeJS.
2. Attempt to monkey patch undici

Additional context

1. Cloudflare implemented .getAll().
2. Node-fetch .raw() implementation
3. Tracking at WinterCG