Should I answer it?
Maybe. Stack Exchange sites exist to provide answers to questions and duplicate questions are no exception. However, the asker will probably be better served by reviewing the existing answers to the question they duplicated and should be directed to it in comments. Duplicate questions are also likely to be closed and/or deleted potentially negating any reputation gained from answers to them.
Should I downvote it?
It depends. Duplicate questions are not necessarily bad; different descriptions of the same problem help future visitors to find the answers they're looking for. However, asking a duplicate question may indicate a lack of adequate preparation prior to asking, which is always grounds for downvoting.
Should I comment?
Yes, with a link. A comment that links to the original question will help the asker by pointing them towards potential solutions to the problem, and alert later readers that the question is a duplicate.
Note that flagging a question as a duplicate will automatically insert a comment to that effect if one's not already present.
Should I edit the question to indicate it's a duplicate?
No. When a question is closed as a duplicate, a banner is automatically placed over the question body with a link to the target as well as the number of answers in the target. Therefore, it would be redundant to edit the link into the question (previously, they were added into the question by the Community user).
Should I tag the question with the duplicate tag?
No. The duplicate tag on Stack Overflow is for programming problems related to duplicate items in collections, not for marking questions that are themselves duplicates.
If I can, should I (vote to) close the question?
It depends. If a question is an exact duplicate, then go ahead and (vote to) close it. However, note that questions may be similar without being exact duplicates:
From the blog:
There could be hundreds of different, related, perfectly valid questions on the same topic. There is no One True Question.
It's rarely this straightforward, however -- usually there are two similar but not-quite-the-same questions, both of which have value for different reasons.
What happens when I vote to close as duplicate?
Or: Why did a comment with my name on it just appear?
A vote to close as duplicate will automatically post a comment with the duplicate link if no one else had voted for that particular duplicate. The user who cast the vote is the owner of the comment and can edit or delete it like any other comment. A comment will not appear if there is an existing comment containing a link to the duplicate.
Once the question is closed as a duplicate, these comments are deleted and the duplicate information is automatically placed above the question body, and "[duplicate]" is appended to the question title view. Questions closed as duplicates are not put in the "[on hold]" state, unlike questions closed for other reasons, but an edit made to the post within five days of closure will still push the post into the reopen queue.
Should I flag it for moderator attention?
It depends. For new questions, a comment with a link to the duplicate question should be sufficient. Enough users with the ability to vote to close will likely see the question and close it without moderator intervention.
For older questions that aren't likely to receive much attention, flag the question for moderator attention and explain the situation including a link to the duplicate question.
What about similar or related questions?
For similar or related questions the following format should be used to edit in links at the bottom of the question:
> [title of question](url to question)