I'm honestly just diving into this now after reading the article, and not a total expert. Wikipedia has a table of a leap second happening across TAI (atomic clock that purely counts seconds) UTC, and unix timestamps according to POSIX: https://en.wikipedia.org/wiki/Unix_time#Leap_seconds
It works out to be that unix time spits out the same integer for 2 seconds.
It works out to be that unix time spits out the same integer for 2 seconds.