<p>I have been reviewing a large number of older questions some months ago (I went through the [pca] tag in its entirety), trying to tidy them up and systematize as much as possible. I have encountered a similar situation a couple of times. @Glen_b gives good general recommendations in his answer, but I think I can make several more points:</p>

<ol>
<li><p>Everybody agrees that it's good to have a "canonical" thread with clearly and concisely formulated question and high quality answers. Try to find a candidate thread among all near-duplicates. Some things to consider:</p>

<ul>
<li>It does <em>not</em> have to be the oldest question;</li>
<li>It should better be well-upvoted (to increase visibility);</li>
<li>It should be concisely (!) and clearly formulated;</li>
<li>If there is an accepted answer, it should better be good;</li>
<li>You can edit the question to make it better.</li>
</ul></li>
<li><p>If such a thread exists, edit it to make it as good as possible. Pay special attention to the title and the tags.</p></li>
<li><p>Check if some other threads contain answers that could be moved into this canonical thread. Is so, flag those for moderator attention and suggest to merge; make sure to explain why (otherwise moderators might not get what you are after and will refuse to act upon your flag). Note that only relatively rarely can answers be seamlessly moved between threads.</p></li>
<li><p>If the original answers and the merged-into answers together still do not satisfactorily answer the question, answer it yourself!</p></li>
<li><p>Now other threads can be closed as duplicates of the new "canonical" one. Go around and vote to close. Sometimes questions might already be closed as duplicates of some other question; in this case I find it helpful to flag them for moderator attention suggesting to change the duplicate link. If you convincingly explain why you suggest that, moderators will do that. I had to use this option a couple of times.</p></li>
<li><p>In some cases, none of the threads is good enough to be canonical. In this case, <strong>consider writing a perfect question and answering it yourself.</strong> If you do it very well and your Q&amp;A gets well accepted and upvoted by the community, you can then vote to close all other (older!) questions as duplicates. I had to do this once, leading to more than a dozen of older questions being now closed as duplicate of my new Q&amp;A. In some cases these questions were already closed as duplicates of one another, so I had to change all that (via moderators).</p></li>
</ol>
