I'm not alone in this, also happened to my friend, and also posted to Reddit:
https://www.reddit.com/r/techsupport/comments/1u7h672/browser_asking_me_to_select_certificate_is_it_a/
Seems like somebody definitely misconfigured something somewhere. The question is then was this the direct result of the misconfiguration, or someone abusing a misconfiguration. Client certificates can be used to identify people so this is a good way to mass collect certs. This data could also be used to analyse client certificates that are around.
a_t48•1h ago
emilfihlman•1h ago
What was also interesting is that it prompted me to select a certificate that was definitely not for that page, ie. it accepted any and all certs, not just ones that are for a certain page (I have incus web generated certificates that should be only for that domain and page, which definitely have nothing to do with Google).