下記が脳内に降臨しました。
I would like to tell, as a person born in Japan that is a single country dropped two atomic bombs in the world.
a single country which was dropped と関係代名詞を更に使うのはNGですか？
As a citizen of the only country that has had two atomic bombs dropped on it, I'd like to tell you XX.
この場合、次のように言うと良いですよ。
ーAs a citizen of the only country that has had two atomic bombs dropped on it, I'd like to tell you XX.
「唯一原爆を２つ落とされた国の市民として、XXを伝えたい」
the only country that has had two atomic bombs dropped on it で「原爆を２つ落とされた唯一の国」
ご参考まで！
カナダ 
