How to display all the values ​​of elements lying by one key in std::map?

Question

C++ / C#

How to display all the values ​​of elements lying by one key in std::map? Here I have a container std::map, it contains:
merlok 89-09-23
merlok 66-66-66
merlok 12-34-56
the task is to display all numbers by the key surname, where surname is merlok

#include <iostream>
#include <map>
#include <string>

struct data {
  std::string number;
  std::string surname;
} user;

void add(std::map<std::string, std::string>& surname_book, 
    std::map<std::string, std::string>& phone_book) {

  while (1) {
    std::cout << "Type 'show' to show the map!\n";
    std::cout << "Enter a phone number: (Type '0' to exit!) ";
    std::cin >> user.number;

    if (user.number == "0") {
      system("cls");
      break;
    }

    std::cout << "Enter a surname: ";
    std::cin >> user.surname;

    phone_book.insert(
        std::pair<std::string, std::string>(user.number, user.surname));
    surname_book.insert(
        std::pair<std::string, std::string>(user.number, user.surname));
  }
}

void search_by_surname(std::map<std::string, std::string>& surname_book) {
  std::string surname;
  std::cout << "Enter the surname to find: ";
  std::cin >> surname;

  std::map<std::string, std::string>::iterator it = surname_book.begin();

  if (it == surname_book.end()) {
    std::cout << "Not found!\n";
  } else {
    std::cout << "Founds:\n";
    for (it; it != surname_book.end(); it++) {      //вывод всех значений по 1 ключу
      std::cout << it->first << std::endl;
    }
  } 
}

void search_by_number(std::map<std::string, std::string>& phone_book) {
  std::string number;
  std::cout << "Enter the number to find: ";
  std::cin >> number;

  std::map<std::string, std::string>::iterator it = phone_book.find(number);

  std::cout << "Founds:\n";
  if (it == phone_book.end()) {
    std::cout << "Not found!\n";
  } else {
    std::cout << it->first << " " << it->second << std::endl;
  }
}

int main() {
  std::map<std::string, std::string> surnameBook;
  std::map<std::string, std::string> phoneBook;

  while (1) {
    std::cout << "Type '0' to exit!\n";
    std::cout << "Type 'add' | 'find':\n";
    std::string choice;
    std::cin >> choice;

    if (choice == "0") {
      system("cls");
      break;
    }

    if (choice == "add") {
      add(surnameBook, phoneBook);
    } else if (choice == "find") {
      std::cout << "1# Find by number\n";
      std::cout << "2# Find by surname:\n";
      std::cout << "Enter a number! ('1' | '2')\n";
      int ans = 0;
      std::cin >> ans;

      switch (ans) {
        case 1:
          search_by_number(phoneBook);
          break;
        case 2:
          search_by_surname(surnameBook);
          break;
      }
      system("pause");
      system("cls");
    }
  }
  std::cout << "Program finished!\n";
}

Answer 1

EMNIP, std:map cannot have multiple elements with the same key. So just
surname_book.find(surname)

Answer 2

It is necessary to use multimap, and in which the last names will be the keys. Then you can display all phones by last name by going through this map from lower_bound to upper_bound. It looks like where you wrote off about such an idea and was. Otherwise, why do you need two absolutely identical maps in the code (surname_book is an exact copy of phone_book)?