Have you ever had an idea, researched it and then discovered that it already exists in the form of an app, website, physical product, etc.? Did you then get discouraged?
Rather than being discouraged, you should be motivated. Why? You have proof that the idea works. Novel ideas are risky because they have not been proven yet. Competition is not a bad thing. You can work on something that someone else has done, even if they dominate the market and even if the market feels saturated. Games are great examples of saturated markets: the market is crowded but there's always a new way to make a new game.
This has happened to me numerous times for work and personal projects. I brainstorm an idea, see that it exists in some form: a product, an open source project, or some other implementation, and then I get discouraged. For me, this bias came from my ego: I believed I needed to create something novel, and if the idea wasn't novel, then it was worthless.
To summarize, if you really believe in the idea: just do it. You can compete on specific dimensions (performance, feature set, design, etc.). You can also think of this as a test of your skill set: can you produce a version that meets, and ideally exceeds, the market's expectations?
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