I shared a book I was working on in a Facebook group I admin (with 10k members). It was just a book that I wanted to exist and read myself. I published it a week later. Then... little by little, similar books started appearing on Amazon. I didn't market it so much, so while mine was published first, it has way less traction than the others. It's a bit frustrating, but that's just the way the world is. People are leaches.

Like others have mentioned, though, your best bet is to just keep going. Keep pushing updates. Try to get reviews. Chances are, if they're copying your app, they're copying other apps. They'll never be able to keep up!