We actually had this for a while. Writing the code wouldn't be too hard, but (AFAIK) it would require a paid subscription to access an API from one of the anti-plagiarism services. I suspect that it would be somewhat expensive to keep up with the posting volume.

I would like to see it, but someone would have to come up with the funding.

Also, it came with its own problems, because people were (IMO) overly aggressive with the downvotes when posts got tagged. It didn't always get it right, and even when it did, there are edge cases where it might be appropriate to copy things. Having the information is good, but the response needs to be proportionate.