Why are some questions marked as duplicate?
The fundamental goal of closing duplicate questions is to help people
find the right answer by getting all of those answers in one place.
I prefer marking as Duplicate early, and then reopening if it doesn't meet the askers requirement. I think most of what we mark as duplicate is accurate, and even many where the asker disagrees it comes down to how they're read/understood the answer in the earlier question and how to apply it to their situation.
How should duplicate questions be handled?
Questions may be duplicates if they have the same (potential) answers. This includes not only word-for-word duplicates, but also the same idea expressed in different words.
It is easy enough to open again if further comment/explanation/clarification shows that it is not a duplicate. As a community I believe we are fairly quick to reopen when necessary.
If we delay while waiting for confirmation then we end up with answers from others, and Q&A duplication.
I would like to think that moderators and other reviewers actually do take the time to review the questions before they make a decision, rather than just closing without thought. The reopening process can be as quick as the closing process. An edit to a question goes into the review queue - the question itself doesn't need to be generating that steady flow of traffic to get the votes.
Sometimes people get it wrong, or the original asker is getting a different result as seen in the duplicated question, and the only way to know that is for additional information to be added to clarify. This is where a user genuinely believes the question is duplicated, and I see no problem with marking that as a duplicate. If you are unsure you can always request clarification first.
If you see something that you think is not a duplicate, hit the Reopen or Flag to put it in a queue, and add a comment.
Also in How should duplicate questions be handled?
Someone says the question is a duplicate, but I disagree!
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
If you think a question is related, but not a duplicate, it's a good
idea to edit the question to add a note explaining the difference. You
can use wording like
Unlike What color is the sky on Titan?, I'm asking about a planet rotating around a brown dwarf.
If you're the author, edit your question to explain how it differs. If
you see the question in the close review queue, you can vote to leave
open. If the question is closed and you have the privilege to vote to
close, you can vote to reopen it.