Authoritative position of duplicate HTTP GET query keys

I am having trouble on finding authoritative information about the behavior with HTTP GET query string duplicate fields, like

http://example.com/page?field=foo&field=bar 

and in particular if the order is kept or not. Most web-oriented languages produce an array containing both foo and bar associated to a key "field", but I would like to know if authoritative statement exist (e.g. on a RFC) about this point. RFC 3986 has a section 3.4. Query, which refers to key=value pairs, but nothing is said on how to interpret order and duplicate fields and so on. This makes sense, since it's backend dependent, and not in the scope of that RFC...

Although a de-facto standard exists, I'd like to see an authoritative source for it, just out of curiosity.

There is no spec on this. You may do what you like.

Typical approaches include: first-given, last-given, array-of-all, string-join-with-comma-of-all.

Suppose the raw request is:

GET /blog/posts?tag=ruby&tag=rails HTTP/1.1
Host: example.com

Then there are various options for what request.query['tag'] should yield, depending on the language or the framework:

request.query['tag'] => 'ruby'
request.query['tag'] => 'rails'
request.query['tag'] => ['ruby', 'rails']
request.query['tag'] => 'ruby,rails'

I can confirm that for PHP (at least in version 4.4.4 and newer) it works like this:

GET /blog/posts?tag=ruby&tag=rails HTTP/1.1
Host: example.com

results in:

request.query['tag'] => 'rails'

But

GET /blog/posts?tag[]=ruby&tag[]=rails HTTP/1.1
Host: example.com

results in:

request.query['tag'] => ['ruby', 'rails']

This behavior is the same for GET and POST data.