You’re right. People can’t have infinite ideas. But simply having the same concept of a different meme doesn’t make it a repost. If it has very similar text, the exact same template, and very little else added, that’s a repost.
It doesn’t take very long to check if someone made the same meme you plan on making.
It’s like speeding. You don’t know you are, but you’re still doing it, and when you get called out for it, you learn to check.