"One," usually. It can also mean "we." It is conjugated in third person. http://french.about.com/od/grammar/ss/subjectpronouns.htm Have a lingot. You're welcome.
I know that "fille" and "garçon" should have been in one of your very first lessons. Although, "appeller" would have been later. Maybe they were teaching it to you again because you didn't quite remember? I have no clue, it sometimes happens to me too but usually I find out that I actually did know it before I had just forgot.
If you do a strengthening exercise early on it will throw some advanced sentences at you. And every language I've tried throws new words in during strengthening exercises with no indication that it is new.
I actually tried the strengthening exercise for French after doing Basics 1 and got all advanced sentences. I guess duo expects us to not strentghhen until later.