I have heard tales of fabled user interface elements that can be used to mark questions as duplicates, or vote for questions to be duped by others. After scouring the question page, this UI is either not there for a user of my rep level, or is harder to find than the Illuminati. Please advise.
When you have a reputation of 3000 or more, you gain the ability to cast close votes. When voting to close, you select a reason, and one of those reasons is "exact duplicate".
Until you get to 3000, you can flag a question as a duplicate. Flagging requires a scant 15 reputation. Click the
flag link, just underneath a question's tags, and select it is a duplicate. Such a flag is not binding, but brings the question to the attention of 3k+ users who can cast their own close votes.
For users with low reputation, the following rules apply:
- You need at least 15 reputation to flag anything.
- You need at least 50 reputation to flag a question as a duplicate (or close for another reason).
This means that if you have more than 15 rep but less than 50, you'll only have the following options when you click
flag under a question:
- it is spam
- it is offensive, abusive, or hate speech
- it is very low quality
- other (needs ♦ moderator attention)
Notice the lack of an "it is a duplicate" option. This is because, as Esoteric Screen Name pointed out in his answer to this question,
... flagging a question as a duplicate automatically creates a "possible duplicate" comment, and you do not have enough rep (50) to earn the "comment everywhere" privilege.
So if you have fewer than 50 rep, searching for a way to mark as duplicate is futile – you haven't yet earned that privilege.
Note: the above information may not apply to all Stack Exchange sites because they may have different policies regarding privileges. For example, here on Meta Stack Exchange, you only need 15 rep to unlock the ability to flag as duplicate because all users have the "comment everywhere" privilege (required rep: 1).