Yup, you can implement the whole ordeal in C++ like this:

  int main(int argc, char** argv) {
    std::vector<std::string> strings{
      "apple", "cAt", "cat", "Dog", "apple", "dog", "Cat",
      "Apple", "dOg", "banana", "cat", "dog", "apple",
    };
  
    strings.erase(
      std::remove_if(
        std::begin(strings),
        std::end(strings),
        [seen = std::unordered_set<std::string>{}](std::string_view str) mutable {
          std::string lower = "";
          std::transform(std::begin(str), std::end(str), std::back_inserter(lower), ::tolower);
          if (seen.find(lower) == std::end(seen)) {
            seen.insert(lower);
            return false;
          }
          return true;
        }
      ),
      std::end(strings)
    );
  
    for (auto& str : strings) {
      std::cout << str << "\n";
    };
  }