You are assuming that the name as it is in Italian today has always been the same and it isn’t. Both Milano and Mailand are linguistic descendants of the name whichever people who first set up shop there spoke and decided to call the place. And that wasn’t anywhere close to modern Italian. They are both valid.

English ditches the o and has Florence on the books as well. Geographical names follow no logical rule. Most are just historical accidents, some historical crimes. This is more in the former category if you ask me.

Cologne, Munich, Brussels, Naples, The Hague … It’s everywhere.