They’re people who copy what you do. It can be flattering, and completely annoying.

If someone copies your idea for a painting, that can be either flattering or annoying. If what they do  turns out to be good, you can be proud of yourself for having that idea and making the other person wanting to do it too. But if their painting turns out to be better than yours and they used your idea, then that’s annoying.                      
If someone copies what you wear, that’s usually annoying. If people don’t notice but start wearing that as well, you have just set a new trend. But if people tell the person who is copying that they look better than you even though you’re the person who started wearing that, I don’t blame you for wanting to scream.

Those are just examples of what can flatter and annoy you about copycats. If you find it flattering, then don’t say anything. If you find it annoying, then you should tell the person who’s copying you that it’s bothering you and ask them to stop. If they say that they’re not copying you and that you’re copying them well, that’s hard to live with. It’s better to drop the conversation if that happens, because it can make things worse.