#include <unordered_map>

vector<string> find_duplicates(DataGenerator& generator) {
    /*
     * Find duplicates in the given data.
     *
     * The `generator` provides a forward iterator over strings
     * for traversing the data, so it can be iterated for example
     * using a `for` cycle:
     *
     *   for (const string& item : generator) {...}
     *
     * The `generator` can be traversed multiple times.
     *
     * The goal is to return a vector of duplicated entries,
     * reporting each duplicated entry only once.
     */
    return vector<string>();
}