C++:
the simple tests succeed, but when I press the 'attempt' button I get this error:

In file included from main.cpp:7:
./tests.cpp:6:8: error: redefinition of 'duplicateCount'
size_t duplicateCount(const std::string& in)
       ^
./solution.cpp:4:8: note: previous definition is here
size_t duplicateCount(const std::string& in)
       ^
1 error generated.

(@-@')

I guess because isize has half of the range of a usize for n>0, so you should avoid an integer overflow when the input is 100.