I have heard tales of fabled UI elements that can be used to mark questions as dupes, 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
For users with low reputation, the following rules apply:
This means that if you have more than 15 rep but less than 50, you'll only have the following options when you click
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,
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).