Is downvoting a duplicate (but otherwise sound) question a good thing to do? It seems to me that it unnecessarily alienates new users (who are probably the most likely to ask a duplicate question.) This question: https://tex.stackexchange.com/q/21858/2693 got a down vote, despite my comment and a couple of close votes.
Thanks Alan for bringing this up!
Please don't downvote a question just because it's a duplicate. It can happen that a duplicate is even better written than the earlier question. Only downvote if the question is bad or unclear, as usual.
Especially with new users it can happen that they did not find the earlier question. Then it would be great to post a nice comment pointing to the duplicate, that's friendlier than just voting to close without a word.
In addition what Stefan already stated:
Especially new users shouldn't be down-voted (especially not below -1!) because this repels them from this site. Point out the duplicate with a comment in a nice way and maybe also politely point them to the search.
Also, as Jeff mentioned already (somewhere else), there is such as thing as good duplicates (and most duplicates are actually good ones). Once marked as such they point to the other question and therefore maximize the chance that people find it, even if they use other search terms.
Might it be possible to add to the WishList the facility for a suitably authorised person to merge a duplicate with the original, reopening the original if closed, in case further answers shed new light? This would notify the OP of the dupe that it was a dupe, and reduce the number of duplicate hits in subsequent searches.