I passed the PSM-I a couple of months ago and today I passed the PSPO-I. Woohoo:) From everything I read, I expected there to be a lot of overlap, and there was. But this had me wondering, why Scrum.org would have two certifications in different paths with so much overlap. Would it make more sense to have a Scrum Foundations Certification that is a prerequisite to all paths, rather than just duplicate so much of the same across level 1 certifications?
I would guess because they are targeted to different roles, so people don't need to take both certifications? E.g. if you're a Product Owner you can just take the PSPO certification, and if you're a Scrum Master you can just take the PSM.
Like what Ben called out, each cert is targeted for a specific role. While it is suggested, and dare I say beneficial to take both; it's not necessary or expected. Both roles need to know the basics of Scrum to be successful in their respective roles, hence the "overlap".