A downvote is supposed - metaphorically - to mean that:
The poster did something bad/wrong, and needs to be sent a message to stop doing it
The post is bad and needs to be signalled to the readers to be ignored.
In my opinion, downvoting a question merely because it is duplicate (with caveats listed below) goes against both of those ideas:
If the poster could not realistically (see below) know it's a duplicate, they didn't do anything wrong. Their actions were not wrong and should not be punished even if the content deserves to be closed.
Duplicate questions - if properly identified and closed as dupes - are sometimes/often not really "bad" content. If the question can be worded in more than one way, a dupe is possibly even GOOD content, since it is likely people would be searching for the information in both of these possible ways. It should still be closed so the answers are all in one place, but it shouldn't be marked as "bad" via downvotes.
And since VTCs as duplicate clearly mark the question as a dupe, there's no need for using downvote mechanism to mark the question as "needs to be ignored".
============= Main proposal =============
As such, I would like to propose the following simple guidelines/rules on downvoting a question when the only problem with the question is that it is a duplicate:
If the question is identifiable by StackExchange SFF software as a duplicate when posting (e.g. it shows up in a list of "Questions that may already have your answer" when typing it up; or at the top of "Similar Questions" on the right when composing) - then it's a bad question that you didn't put effort in. Fair game for downvoting on the "no research" grounds.
EXCEPTION: When OP explicitly notes in the Q or comments that he saw possible duplicate lists, evaluated them and gives good reasons why they were not satisfactory to find the answer. Even if the OP is wrong and they really ARE duplicates, the OP did the research and this should not be penalized merely for being wrong.
If the question is identifiable by obvious search as a duplicate (in other words, you type the subject of the question into Google and an existing SFF.SE question that can reasonably assumed to be a duplicate pops up in top 10 results). Fair game for downvoting.
EXCEPTION: Same as above. If OP explicitly mentions that they did this, found X, Y and Z questions and were not satisfied, they DID their research and should not be punished by downvoting.
If the question is bad for OTHER reasons that deserve a down-vote, and happens to be a duplicate, you may downvote but please clearly communicate via comments why you are downvoting.
In any other case, do NOT downvote a duplicate question. Either simply vote to close, or if not enough rep, flag for closing, or post a comment linking to the possible dupe.
To re-iterate, merely commenting "this is a duplicate of X" without explaining how X could have been found by original poster should NOT be valid grounds for down-vote.
Just because Slytherincess probably remembers almost every SFF Harry Potter question by heart, it should not be expected of every user.
If the community accepts these rules, I even have a suggestion for a possible enforcement mechanism:
If you DV for "no research" but don't leave a comment explaining how the OP could have found the duplicate question (I read that question 6 months ago and remember it is NOT a valid explanation), moderators - assuming they have that power - will be petitioned on meta or flags to remove that downvote.
If you DV for "other bad reasons" on a duplicate question, but leave no comment explaining why, it will be assumed to be a previous case and will be also petitioned to have the vote removed.
We ask StackExchange to develop a script that deletes all downvotes from a given question (akin to serial downvote reversal one); no matter who cast them; triggerable only by a moderator. Just to be clear - this is not about removing specific user's votes. It's about clearing all of them from a given Q; if meta discussion of it shows that it doesn't deserve downvoting.
To be crystal clear - "doesn't deserve downvoting" here is not a generic "any question's downvotes can be removed by Meta discussion" but very specifically applicable to downvotes cast on duplicate questions because a large amount of users are not familiar with the guidelines of when a duplicate question should be downvoted - in this case, Slytherincess was not aware of it, and she's not exactly a newbie. So were others. It's a bit of a niche topic, but one that has clear potential to harm and drive away new posters.