:::::It already was mentioned, and it wasn't a big deal. Now it's mentioned twice, and it is a big deal. 

 :::::Look, the fact is that Chopin considered himself a Pole rather than just a guy from the Grand Duchy of Warsaw, just as Beethoven considered himself a German rather than just a Kölner. You're retroactively projecting a political notion of "citizenship" onto people to whom such a thing simply wouldn't occur, and in doing so, you're confusing the 20th/21st century political entity of "Poland" and "Germany" with the 18th/19th century notion of "Poland" and "Germany" as an identity, something that was not bound by mundane geography or fleeting political situations. That the Britannica makes the same mistake does not mean that it isn't a mistake. 

 :::::Furthermore, the fact that the Duchy was annexed by Russia and effectively dissolved long before Chopin was even old enough to understand what was going on (unless you'd like to argue that he was a political savant at five years old, in addition to being a musical prodigy) should tell you just how much its existence meant in his life.