You can click on "Show Kata Test Cases" and that should display all of them.
I think this is the one failing?

    #[test]
    #[allow(non_snake_case)]
    fn test_lowercaseaaAb() {
        let lowercase = "abcdefghijklmnopqrstuvwxyz";
        assert_eq!(count_duplicates(&format!("{}aaAb", lowercase)), 2);
    }

Ahh, good idea on initializing the string with an equal length to the input!