reread the description for conditions that the question requires

maybe capitalization is the reason?

Ignore capitalization when determining if a character is a duplicate.

It's already there in the description.

Hello everybody, I really like the Idea of this kata. However, there is one big problem that I faced: When looking at the tests, the creator of this kata implies, that 'S' is equal to 's'. So all uppercase characters are to be treated the same as lowercase characters. From my point of view, that is a wrong implication, since 'S' and 's' have different ascii codes! Change this please or make it clear in the description of the kata. Aside from that, great kata:)