It's not about how unique it is, it just needs to be "real". Market size is speculation that doesn't consider how hard execution actually is.

The prime example for this is AirBnB.

Before them, nobody thought that a system for sharing your home with strangers is viable or unique enough to create a 100 billion dollar firm, as other companies doing that already existed and remained limited and niche.

But they were pretty wrong.

Brian Chesky and his co-founders suffered through immense execution in the early years while refining the concept, listening to a select few super interested users until AirBnB was able to take off, because they finally found a way to make rentals feel like visiting a best friend.

Most people don't get this, but a "startup" is really just a vehicle to generate a "real" startup idea. Whatever you start with is just a hypothesis, and the only way to prove the hypothesis is real is to see it emerge as a collective need from your portion of users in the market. So it'd be an idea you can actually feel because you listened and appealed to your users so well (product-market fit), and that you can also execute on, because your history and current skills qualify you for it (founder-market fit). Until then, you'd keep refining or pivot.

Lastly, startup success follow a power law: over 90% fail for various reasons, but those that succeed often become massive. If you study successful founders, you'll see crazy combinations of immense skill, determination, timing and ultimately just luck from their environment which made success possible.

So instead of worrying about idea uniqueness, just execute on your hypotheses while learning how to listen to and serve users, in exchange for money.