So I was wondering how to find out if a story you thought thought of is already written

I came up with a story a while back and was planning to write it and then i read a story and some of the stuff sounded like what I was thinking

This also happened to my cousin who was going to write one and she told me the idea and then she started writing and a few days later i found one that sounded so close to it!

I dont wanna make someone feel like I plagarized them cuz im against that and i dont wanna ask for a poster and end up having to take it down, but how can i know if my story exists? can anyone help me