Circular Header Problem

//I really don't know how to manage the header files so this can compile fine. Database has a vector of type User, and user //should access that vector...
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#pragma once
#include "Utility.h"
#include "User.h"
#include "Course.h"

class Database
{
    std::vector<Course> allCourses;
    std::vector<User> allUsers;

    void printUserDatabase()noexcept;
    void printCourseDatabase()noexcept;
    void loadUserDatabase()noexcept;
    void loadCourseDatabase()noexcept;
};
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#pragma once
#include "Utility.h"
#include "Database.h"
class User: public Database
{
public:
    User()noexcept;                                                                //defaultni konstruktor
    User(std::string, std::string, std::string, std::string)noexcept;              //ime, prezime, username, sifra -> type automatski
    User(std::string, std::string, std::string, std::string, std::string)noexcept; //ime, prezime, username, sifra, type

    std::string classType()const noexcept; //vraca nam tip objekta
    std::string getName()const noexcept;                                                  //getter za ime
    std::string getPassword()const noexcept;                                              //getter za password
    std::string getSurname()const noexcept;                                               //getter za prezime
    std::string getUsername()const noexcept;                                              //getter za username

    virtual std::string checkType()const noexcept;                                       //vraca tip koji je sastavni dio/atribut korisnika
    virtual void loginMethod()noexcept(false) {}; //virtualna metoda za login
    virtual void menu()noexcept;                                                   //menu je virtualan jer svaka izvedena klasa ima svoj menu

    void addFriend(User&);                                                 // dodavanje prijatelja na osnovu username-a, gleda se samo username kod Usera
    void checkRequest()noexcept;                                                   //metoda koja omogucava pregled dobijenih zahtjeva i njihovo odobravanje/odbijanje
    void sendMessage() noexcept;                                                    //metoda za slanje poruka na osnovu username-a
    void sendRequest() noexcept;                                                    //metoda za slanje zahtjeva na osnovu username-a
    bool checkFriend(std::string) noexcept;                       //provjerava da li je korisnik sa datim username-om prijatelj
    bool checkUsername(std::string username)const noexcept;
protected:

    std::string myMessagePath(std::string, std::string)const noexcept; //kreiranje imena fajla za inbox izmedju dva korisnika
    std::string myPath();                                //
    std::string password;             //password korisnika
    std::string name;                 //ime korisnika
    std::string surname;              //prezime korisnika
    std::string type;                 //tip korisnika
    std::string username;             //username korisnika

    std::vector<Course> enrolledCourses;         //lista kurseva na koje je korisnik prijavljen ili na kojima predaje
    std::vector<User> friends;        //lista prijatelja
    std::vector<User> requests;       //lista zahtjeva za prijateljstvo

    void header()const noexcept;                                                         //sluzi mi samo za ljepsi ispis prilikom ispisa tabele sa korisnicima
    void loadEnrolledCourses()noexcept;                                    //ucitavanje kurseva na kojim predavac predaje ili kurseva koje student pohadja
    void loadFiles()noexcept;
    void loadFriendsDatabase()noexcept;                                            //ucitavanje prijatelja iz fajla u vektor
    void loadInbox()noexcept;                                                      //ucitavanje poruka na osnovu username-a
    void printEnrolledCoursesDatabase() noexcept;                                   //ispis mojih kurseva
    void printFriendsDatabase() noexcept;                                           //ispis datoteke gdje se nalazi lista prijatelja, fajl this->usernamefriends.txt
    void updateUserFiles();                                        //updateovanje fajlova u slucaju promjene

    friend  std::istream& operator>>(std::istream& dat, User& a)
    {
        dat >> a.name >> a.surname >> a.username >> a.password >> a.type;
        return dat;
    }
    friend  std::ostream& operator<<(std::ostream& dat, User& a)
    {
        dat << w_ << a.name << w_ << a.surname << w_ << a.username << w_ << a.password << w_ << a.type;
        return dat;
    }

    friend bool operator==(const User& a, const User& b) {
        return  a.getUsername() == b.getUsername();
    }
    friend bool operator<(const User& a, const User& b) {
        return  a.getUsername() < b.getUsername();
    }
    friend bool operator>(const User& a, const User& b) {
        return a.getUsername() > b.getUsername();
    }
    friend bool operator<=(const User& a, const User& b) {
        return a.getUsername() <= b.getUsername();
    }
    friend bool operator>=(const User& a, const User& b) {
        return a.getUsername() >= b.getUsername();
    }

};