not all things are an adaptation to evolutionary pressures

sometimes there are just single nucleotide polymorphisms that change things.

A second spleen is probably just a developmental defect and not genetic in any way.

Well it functions in both blood filtration and for blood/immune cell storage, both of which are critical for staying alive and reproduction. But when it comes to evolution, things aren’t always straight forward. Maybe there was a entirely different function which had a bigger positive impact on overall fitness and since the spleen situation didn’t decrease fitness, it too was past along.

I doubt this is the result of pressure, it would be much more common if it were, at least in a certain region or within a certain ethnicity.

But let’s roll with it and create a fictional pressure. You could have something like a disease that slowly degrades the spleen making very few people reach reproductive maturity. Let’s imagine that having a second spleen means they are much more likely to reach this age, and there’s how you would make the second spleen more common.

Honestly I doubt you could ever have a disease that specific but I can’t come up with a better idea atm.

All humans start with 2 spleens, they fuse in utero. When this process fails to complete you end up with an accessory spleen or “splenunculus”. There is no functional defecit, you can live happily with an accessory spleen.