This is in part a public service announcement and in part an invitation for discussion (see also the comments on Martin Tapankov's nomination).
First the announcement: everyone with at least 3000 rep can cast close votes on questions. In particular, if one thinks that a question is an exact duplicate, one can mark it as such by using the close button (it will then ask for a link to the duplicate question). This will not immediately close the question—it only closes when five people have cast close vote. It is not considered as an offense to the poster to cast a close vote (but you might want to add a comment in addition to the automatic one).
Now for the discussion: How aggressive should we be in closing posts as duplicates? I have noticed several times now that someone posts a comment pointing out a duplicate that gets upvoted several times, but (almost) nobody casts a close vote. Is this because people feel that we shouldn't close duplicates, or because they don't know that they have the power to do so?