YC companies also copy other YC companies. It happens all the time. Personally I would prefer that the world didn't work this way, but it's just how the game is played.
There are a lot of people in the tech world with the right connections, access to capital, and execution skill, but who either aren't creative enough to come up with an innovative product, or prefer to be the nth-mover in an already-validated market to the risk of trying to establish a new market. If they see that you have a good idea but aren't executing/growing/raising money at an A+ level, they will swoop in and copy your idea and then pretend that they were the first to come up with it.
There's nothing necessarily wrong with this apart from (imo) being kind of lame on a personal level. The consumer ultimately benefits from the competition, and you have only yourself to blame really for creating an opening by not executing quickly and effectively enough. It's capitalism. All the solutions to this 'problem' I've ever heard proposed--software patents and the like--create far worse problems for people who want to innovate.
If you're working on a new idea and are seeing real traction, you just need to expect this and factor it into your strategy. This is why so much emphasis is placed on having a 'moat' of some kind that is difficult to reproduce. If you're seeing success and it's easy to copy you, someone's probably already working on a clone.