YC has funded and continues to fund a lot of payment startups. More generally: YC funds close to a thousand startups a year at present—obviously it's impossible to avoid funding competitors in such a model, nor would that be in keeping with YC"s founding principles, one of which is to try to fund as many good startups as possible (https://www.ycombinator.com/principles/). But you can have good practices around it, such as not playing favorites, not giving inside information about one to another, reminding founders that startups mostly don't die because of competition, big markets are big enough to support multiple players, and so on.