I have no idea how it works in other countries, but in Poland there are publicly accessible `.csv` files with data about names and surnames in the PESEL system (the ID assigned to people with Polish citizenship) since its creation. A double surname would be marked as a separate entry. If something like this exists in other countries, checking the rarity of a combination of surnames would be quite easy.

I'm the only person in the UK with my sequence of names, they told me so when I was getting my covid jab and didn't have my NHS number on me so they had to find me by name and match to my photo ID.

French immigrant mother so French first name and middle name, and then an uncommon British (Welsh I think, dad's parents didn't learn English until they got to secondary school) second name.

Google, Facebook, Twitter are some indications. Not 100% guaranteed though.