Duplicates when inserting into std::set

I tried to use std::tuple as an element of std::set where std tuple consists of std::string and a bool (for this example only, in normal use case the tuple consists of 3 elements). The string of the tuple shall be unique, therefore I created comparator that would sort the elements on the first element of the tuple. However, when inserting into aforementioned set I do get duplicate elements in the container. I cannot see where I am making a mistake. Thanks in advance for suggestions!

#include <set>
#include <functional>
#include <iostream>

using namespace std;

int main() {

    using tuple_t = std::tuple<std::string, bool>;
    std::function<bool(const tuple_t &, const tuple_t &)> tupleCmp =
            [](const tuple_t & l, const tuple_t & r) -> bool {
                return static_cast<bool>(std::get<0>(l).compare(std::get<0>(r)));
            };

    set<tuple_t, decltype(tupleCmp)> s(tupleCmp);
    s.insert(make_tuple("ahoy", false));
    s.insert(make_tuple("this", false));
    s.insert(make_tuple("creates", false));
    s.insert(make_tuple("duplicates", false));
    s.insert(make_tuple("ahoy", false));

    for (const auto & i :  s){
        cout << get<0>(i) << endl;
    }
    auto debug_stop = 0;
}

Debugger snippet

Output:
ahoy
duplicates
creates
this
ahoy