Plagiarism was always problem on Steemit and some people just don't feel any shame while doing things that can harm other people's work and reputation. There are many different way which you can plagiarize others, you can copy their text, steal their pictures and images and present them as yours... and in many cases, these plagiarists will not be noticed and marked as thieves. But is stealing an idea considered as plagiarism? Well, that's something to discuss because the idea doesn't have to be explained the same way by original author and the one who was inspired by original author and made his own work.
So it is very hard to claim that you are the owner od an idea. You have to be a pretty genius scientist and have very advanced brain to claim that you "own the idea". Because you're so smart and nobody ever had that idea...
One starts with an idea and posts about it! If his post was successful, it will probably inspire few of his followers to start writing about same things. Is that plagiarism? NO! You just opened the door for others, let them in and now they act like they need to spread the word... if you have this sort of followers who will take your ideas and write about same things but using their own words, that can mean only one thing: that you have awesome ideas! But in almost all cases they will not name you as an original author of that idea! Because they use their own words, you can't do nothing!
If you find your own ideas in some texts of users that do not follow you, they probably had the same ideas like you but explained them differently. But when someone follows you, comments your posts, supports your ideas and then, two-three days later, you see very similar title with very similar text but with completely the same message... and no one mentioned that you inspired them for that!