Hello!

Takosabi is right. It doesn't matter if you already got a kind of ninja or not for you to be able to get one again. The chances you have will never change even if you already got one or two. And Mission Boxes doesn't work like Seal Scroll Treasures if you already have one kind of ninja and you got him again from the box then you will get full 80 fragments of it. I experienced getting Hashirama Senju [First Hokage] 3 times a long time ago as well as getting Tenten [Great Ninja War] 3 times (just giving as one of the examples but it is from Lucky Ninja Pack). Hope you understand it, thank you.