How to fix combination Dutch domain - English uri
My favourite way of dealing with 404 like that is to use module like Search 404:


  Instead of showing a standard "404 Page not found", this module performs a search on the keywords in the URL, e.g. if a user goes to http://example.com/does/not/exist, this module will do a search for "does not exist" and shows the result of the search instead of the 404 page. This should help retain visitors coming in from old URLs linked from other sites or from search indices.


That way user will see a link to an article on a proper domain, if he will enter bad URL.

To prevent creation of "interesting" links it's worth to use Global Redirect - with it even if you will have temporary misconfiguration, users and bots (including Google bot) will get redirected to content's main URL, and that's what will get indexed.

Last but not least, be sure to always set canonical URL for your content, if you care how it looks in search results.

I'm afraid you will never know what misconfiguration caused this in the first place, unless it still occurs. Things to check would be:


Canonical URL - are you sure it does not direct everyone to one single domain?
Web Archive - maybe they have a snapshot from time when your site was actually displaying something under these URLs. If so, you could use backups to unpack a copy on a dev machine, and examine configuration that made this.