I have flagged a question saying it should be closed , but it says
declined - a moderator reviewed your flag, but found no evidence to support it
However, the question is closed as I claimed.
What's the problem?
You used a custom flag message stating:
This may very well be the case, but moderators are not Google search. If, as you state, there are lots of possible duplicates which you are aware of, you could have used the "exact duplicate" moderator flag and showed us which post it was a duplicate of.
While moderators are super-human, we aren't omniscient. That said, aren't expected to know the entire breadth and width of all of the questions on Stack Overflow.
My assumption (and I am not speaking for the moderator here, it wasn't me though) is that you only pointed a finger without any evidence to support the claim. With other flags (OT, NC, NARQ) the post itself is the evidence, but in the case of duplicates, we need something more than "a duplicate exists", tell us what it's a duplicate of.
However, the question is closed as NARQ. Without knowing what exactly the question was a dupe of, looking at the question on its own, it's obvious that it is not a good fit for Stack Overflow in it's current form, so I assume it was closed on those grounds.
As a general guideline, if a flag leads to action on a post, the flag should generally be marked as "helpful" even if the flag doesn't align exactly with the action taken; the general idea being that you alerted us to something that was wrong, and it was indeed, helpful.
However, sometimes, moderators are flooded with similar flags from the same user (I'm not sure if this is what happened here) and we will decline a few of them to indicate that the close reason isn't correct.
This is acceptable, in that if a user is going to be very active about flagging, but isn't flagging the content correctly, it helps make the entire process more efficient if the flags that are given are generally more in line with what's actually wrong with the post.
Then, when the user gets the decline and reads the message, they generally flag more appropriately, and it streamlines the entire process for both sides.