<p>The implicit assumption behind the issues raised here seems to be that closing is a form of removing, hiding, or destroying something we do not want.  I would like to suggest that closing, on the contrary, is an act that <em>adds information and value to our site.</em>  To understand this, bear in mind that <em>the threads associated with closed questions remain visible and <a href="https://stats.stackexchange.com/search?q=closed%3Ayes">searchable</a></em>.</p>

<p><strong>Pointing to an off-topic as a duplicate is rare, but it is not inconsistent.</strong>  Although questions can officially be closed for only one reason, often several apply.  Moreover, off-topic threads frequently collect comments (and, on occasion, answers).  Linking to such threads <em>is informative.</em></p>

<p><strong>Closing duplicates of unanswered questions makes perfect sense.</strong>  When the same question appears in two or more places, then for the purpose of deciding whether they are duplicates <em>it does not matter whether neither has an answer</em>.  We want to link them so that all the answers (and, hopefully) comments appear together in a conveniently read, easily searched way.</p>

<p>The proposed solution--do not mark unanswered questions as duplicates--would only cause <em>unanswerable</em> questions to proliferate and degrade our site.  As far as I can see, the only reasonable alternative is to <em>delete</em> duplicate questions.  It is rare for moderators to do that, because duplicates are almost never phrased exactly the same way: they may contain different key words, for instance.  In many cases it might not be evident to a neophyte that two questions are really duplicates, because recognizing that might require understanding the meanings of related technical words or of making a simple mathematical transformation.  <em>The act of linking a question to a duplicate embodies such an insight</em> and itself creates meaning.  This adds value to our site, whether or not either question currently has an answer.</p>

<hr>

<p>There <em>is</em> a problem, however.  It lies with the system's stock phrase, automatically applied when a duplicate is closed:</p>

<blockquote>
  <p>This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.</p>
</blockquote>

<p>This obviously is not true when the duplicate does not have an answer--but there is no way anyone here on CV can modify this text to make it more accurate.</p>
