I think a different, perhaps weird, way of looking at this is that YC looks at the founders first and then market opportunity ("idea") secondly as justification

I think a different, perhaps weird, way of looking at this is that YC looks at the founders first and then market opportunity ("idea") secondly as justification. YC doesn't need a single version of an idea to "win", and it's often stated plainly it doesn't even necessarily value the "idea" behind a lot of these investments but the founders themselves. If you look at it that way, it's no surprise that multiple startups are working at the same market opportunity ("idea"), and that YC actually just wanted the founders themselves regardless of whose execution of the "idea" ends up winning (if any).