I’m not sure duplication of information is a bad thing. I mean, redundancy can help in many cases (if Commons has a temporary downtime an alternate link could back up the same information so the user can still see the image). But having both image and wikimedia_commons link to the same file I agree is quite useless. I’m not sure what’s my favourite chiuse in this case, though. I think that using wikimedia_commons only for categories of page-galleries could be good (although, if there’s a Wikidata item linked that could suffice and provide images from linked category) but I’d use it for files hosted on Commons too.