Dubbin,
When (as KCChiefs9690 points out) it is in the same forum, usually there is no need. If there are already replies in both copies, I at least merge them so there is no need to do that. BTW, if it is a duplicate thread, usually you don't close it, it is deleted, so you don't see it at all.
I suspect what you are talking about is when someone raises a topic, that has been raised a number of times in the past and the topic is closed as duplicate. In that case it is problematic as usually there are several old topics on the subject. Usually those are only closed if the topic tends to start problems (I am sure you don't need amplification on that need) or if the topic was just discussed last week or within the past few days. However, I will endeavor (but I can't speak for others) to find a link if appropriate (not a problem topic) and set it in the closing post.