<p>Sometimes a duplicate question gets an answer before it gets closed as a duplicate. In some cases this answer is either better than the answer(s) in the "main" question, or at least adds something valuable to them. Such a situation can be <strong>very</strong> annoying, because it is not clear what future duplicate questions should be made duplicate of, information gets scattered, etc.</p>

<p>Technically, such a duplicate question can be <em>merged</em> into the "main" one; this copies all the answers into the "main" one, as if they were there all along. Only moderators can perform such merging. <strong>It is not clear to me what guidelines moderators use to decide on merging, and I would be grateful for explanations.</strong></p>

<p>Here is one example: </p>

<ul>
<li>Main question: <a href="https://stats.stackexchange.com/questions/27300">Using principal component analysis (PCA) for feature selection</a></li>
<li>A recent duplicate with a decent answer: <a href="https://stats.stackexchange.com/questions/126305">PCA Feature Selection</a></li>
<li>An old duplicate with a decent answer: <a href="https://stats.stackexchange.com/questions/82992">Using principal component analysis (PCA) for feature selection in regression</a></li>
</ul>

<p>I flagged the recent duplicate for moderators attention suggesting a merge; my flag has been marked as "helpful", but nothing else happened. <strong>What does it mean?</strong></p>

<p>I did not flag the old duplicate, as I am less sure that it should be merged. E.g. the answer there is accepted; I guess in case of a merge, the answerer would loose the reputation associated with having an answer accepted, which is a pity. Still, reputation issues aside, I think it would be more convenient if this all was merged.</p>
