I read a fanfiction that I really love but have recently read a manga that is SEVERAL years older (over 4 years I think) with the same plot, exact same events, same characters with different names (same roles and actions), and even share over 5 same quotes(exact wording on the scanlation available on MangaPark. I’m absolutely sure it is not an accident. It is a featured story on AFF too. I have reached out to the suspected plagiarizer but never recieved a response. While I can write in Japanese I do not know how and if I can report it. I do really like their work so I am hesitant to report it and they wrote a big long “similarities are coincidental” passage on the forward of the story.
My comment has not been deleted but I do see that the author has logged in since I made the comment. I even included the suspected passages in there and the name of the suspected source material.

As somebody who's been plagiarised several times before and also helped to sort out plagiarism cases, I have to say that the first port of call if something has been plagiarised should ALWAYS be to contact the author whose hard work has been nicked and the mods. I've seen (and it's happened to me as well) people contacting the plagiariser first instead, which often doesn't solve anything - quite often the plagiariser will just deactivate and then reactivate at a later date, or sometimes just block the people (and the original author) so that they can no longer see what's going on. The person who has been plagiarised should be the one calling the shots to make sure that it is dealt with correctly. Not to mention that if people take it on themselves to sort out plagiarism when they aren't either in the offending or offended party, that's often when the witch hunts and the flaming starts. I've also seen people assuming that something is plagiarism and giving the person a hard time when they have actually received permission from the original author - which is why it is important to contact the original author first, to ensure that there are no misunderstandings. (Also bear in mind that sometimes people do lie about whether or not they've been given permission.)

In principle I support trying to reduce instances of plagiarism, but the proper mode of dealing with it ought to be 1) inform the author who has been plagiarised (so that they can deal with it as they see fit - if they're approached in the right way, most will be fair approaching the plagiariser rather than going at them all guns blazing) and then 2) report it to the mods (or let the author do so in order to stop the mods from being flooded). Also, to prove that it is plagiarism, screenshots ought to be taken of both works to prove the similarities, and of the dates, and any proof that the plagiariser had read the original (e.g. if they're subscribed to the story/had commented on it etc.).

I am definitely for this. I forgot where I saw this, but there was one fic from wattpad that was posted here because the AFF user "thought it was really funny and wanted to share it," but I didn't see any form of permission at all, which irked me a lot; I should have reported it :-/