I'm actually an expert in Italian history^[citation needed]^ so I can chime in here: the ruins that archeologists are excavating are called Pompe2, but the original city was named Pompe1
Explanation: 'I' is both a Latin letter and a numeral. In Roman numerals, two numbers next to each other add together - as in, 'II' is '2'. This joke is making fun of the fact that ii is both a very common ending for Latin words (something to do with grammar, Latin grammar is a nightmare), and the number 2 (as in, a sequel and not an original).
I was a Latin tutor for a while, grammar was actually my favourite part. I used quizzing my students on the meaning of words to conceal when I wasn't sure myself because I never did get along with memorising vocabulary (though seeing words used does help remember them).
It's mostly because of the suffix -ius /jus/, added to nouns to form adjectives. Both the nominative plural and genitive singular of that suffix are -iī /ji:/, so if you're talking about multiple of that thing or something belonging to that thing, it gets the -iī.
It gets worse in this word though. Classical pronunciation was likely /pom.pej.ji:/, both the first and second "i" were long, or like you had three "i" in a row. And this mess wasn't even originated in Latin, but a Southern relative called Oscan.