This applies to examples of duplicates that are cross-postings by one individual. That is not the case with the original example, but quite often occurs.
If I notice this I usually (should) refer the poster to the advice within https://stats.stackexchange.com/help/on-topic
Please note, however, that cross-posting is not encouraged on SE
sites. Choose one best location to post your question. Later, if it
proves better suited on another site, it can be migrated.
A related issue is when you notice that (essentially) the same question has been posted in a forum outside Stack Exchange. I usually post a link on CV (or SO, where I am fairly active) to the other discussion and underline that it's polite to tell people about cross-posting elsewhere. And vice versa, if I am active at the other place. Other forums may have policies on cross-posting that may apply. Statalist has an explicit policy that you are asked to tell people about cross-posting.
The principles here are simple, but perhaps worth spelling out:
It can be genuinely difficult, even for people familiar with various sites, to know which is the best place to ask a particular question.
It can be a waste of your time and effort if you answer on one forum when there's already a good answer elsewhere. (There might be some gain for the person answering in the fun or even learning that answering a question imparts, but my guess is that most people tempted to answer a question would rather know if the question has been asked, and so possibly answered, elsewhere.)
People interested in an answer will presumably want to know if an answer may exist elsewhere.
Some people regard cross-posting as offensive in so far as the poster is implying that the community can't be trusted to give a good answer and/or that the poster does not care about, or has not thought carefully enough about, the possibility of duplicated effort. I don't go that far personally -- any offence is usually unintentional -- but it's important that posters know that such opinions exist.
I've encountered the argument that cross-posting is a good idea because very few people use all the sites in question. But it's because that is so that there is a need to tell people about it. Cross-posting would only be obvious if everyone constantly visited every possible site in a field, which isn't the case.