And Star Wars was a mash up of Dune & "The Hidden Fortress" (Japanese).
Magnificent Seven is Seven Samurai.
Plagiarism is more than copying the plot. The Dune Film maker lost the case against Lucas over Star Wars.
Almost every Pirate book and film since Treasure Island is a knock off. Stevenson invented the Hidden Buried Pirate Treasure. It wasn't a real thing. He himself gives a thanks to Coral Is.
Almost every Shakespeare was a close copy. Only a couple are really original.
Miss Marple is a knock off of Miss Silver. Poirot (Belgian) of Inspector Hanaud (French written by A. Mason).
https://en.wikipedia.org/wiki/Inspector_Hanaud
Compare Harry Potter with "The Worst Witch" Jill Murphy, "The secret of Platform 13" Eva Ibbotson and Boarding School Stories (Blyton's two series copy of others, see Chalet School and others).
There is nothing wrong with any of that. Though Disney tries to pretend they own all the PD fairy stories they've used. They have ownership of their names of the Dwarves. And people that think they can Trademark or Copyright "Winter is Coming" are greedy fools.
You can't copyright the actual title of a book. Loads of books have the same titles.
Actionable Plagiarism is easier to prove in a scientific paper and really hard with fiction unless the "author" is totally incompetent.