You don't have to credit whoever's idea it was to first write veela fic or mpreg fic, you don't have to credit whoever's idea it was to first write Harry Potter fanfiction, J.K. didn't have to credit whoever's idea it was to write lore about wizards - and isn't there a saying about how every basic story idea has already been written somewhere? Specific things I see crediting for, like spells or potions if they're really creative and developed. For example, I made up a spell that checks for pregnancy and called it Gravidus, because gravida is latin for pregnant. If I saw that somewhere else I wouldn't for a second think of it as plagiarism, because it's really obvious and easy to come up with. Good chance it had been done before me, even. If someone really went out of their way to construct this intricate spell or potion that played into their plot, though, I'm sure they'd appreciate getting credit for it. Eventually, though, if it was popular enough, it would probably just become a part of fanon and down the line people wouldn't even know who to credit anyway.

Complicated question. Obviously don't full-on steal someone else's work or read a story and then write one that totally mirrors it with just the wording switched around a bit, but other than that I'd say give credit when we can, and not get too worked up about little things.