<p><strong>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.</strong> </p>

<p>If I notice this I usually (should) refer the poster to the advice within <a href="https://stats.stackexchange.com/help/on-topic">https://stats.stackexchange.com/help/on-topic</a> </p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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. </p>

<p>The principles here are simple, but perhaps worth spelling out: </p>

<ol>
<li><p>It can be genuinely difficult, even for people familiar with various sites, to know which is the best place to ask a particular question. </p></li>
<li><p>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.) </p></li>
<li><p>People interested in an answer will presumably want to know if an answer may exist elsewhere. </p></li>
<li><p>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. </p></li>
</ol>

<p>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. </p>
