Getting linker error with header file and function definition file [duplicate]

I’m getting this huge linker error whenever I try to run these files.

I have a setup of my main code (main.cpp), my header file (Functions.hpp) and my function definition file (Functions.cpp).

I have made sure that both my main.cpp and Functions.cpp both have the #include "Functions.hpp" at the top.

Main.cpp

#include <iostream>
#include <fstream>
#include "Functions.hpp"
int main() {
  std::cout << "Would you like to sign up? (y/n): ";
  std::cin >> signuptrue;  
  if (signuptrue == "y" || signuptrue == "Y" || signuptrue == "Yes" || signuptrue == "yes") {
    IsSignUp = true;
  } else if (signuptrue == "n" || signuptrue == "N" || signuptrue == "No" || signuptrue == "no") {
    IsSignUp = false;
  } else {
    std::cout << "Invalid entry\n";
  }
  if (IsSignUp) {
    signup();
  }   
  goodLogin = login();
  if (goodLogin) {
    std::cout << "You are logged in\n";
    mainMenu();
  } else {
    std::cout << "You are not logged in, please try again.";
  }
}

Functions.hpp

#include <iostream>
void signup();
bool login();
void mainMenu();
int findGold();
int updateGold(int newgold);
std::string SignUpUsername;
std::string SignUpPassword;
std::string Entered_username;
std::string Entered_password;
std::string Entered_login;
std::string signuptrue;
bool goodLogin = false;
bool IsSignUp = false;
int gold;
int newgold = 0;
int action = 0;
std::hash<std::string> hasher;
std::size_t hashedInput

Functions.cpp

#include <iostream>
#include <fstream>
#include "Functions.hpp"

void signup() {
  std::string EnteredLogin;
  std::cout << "Enter a username: ";
  std::cin >> SignUpUsername;
  std::cout << "Enter a password: ";
  std::cin >> SignUpPassword;
  std::cout << "You have signed up successfully\n";
  std::string combinedInput = SignUpUsername + SignUpPassword;
  std::size_t hashedInput = hasher(combinedInput);
  std::fstream accountFile;
  accountFile.open ("accounts.txt", std::ios::app);
  accountFile << hashedInput << '\n';
  accountFile.close();
}

bool login() {
  std::string line;
  std::cout << "Enter your username: ";
  std::cin >> Entered_username;
  std::cout << "Enter your password: ";
  std::cin >> Entered_password;
  std::string combinedInput = Entered_username + Entered_password;
  hashedInput = hasher(combinedInput);
  std::ifstream accountFile;
  accountFile.open("accounts.txt");
  while (getline(accountFile, line)) {
    std::size_t hashedFileInput = std::stoul(line);
    if (hashedInput == hashedFileInput) {
      return true;
    }
  }
  return false;
}

int findGold() {
  std::ifstream accountFile("accounts.txt");
  std::string line, goldAmountStr;
  while (getline(accountFile, line)) {
    if (line == std::to_string(hashedInput)) {
      std::getline(accountFile, goldAmountStr);
      try {
        int goldAmount = std::stoi(goldAmountStr);
        return goldAmount;
      } catch(const std::invalid_argument& e) {
        return 0;
      }
    }
  }
  accountFile.close();
  return 0;
}

int updateGold(int newGold) {
  std::ifstream inFile("accounts.txt");
  std::ofstream outFile("temp.txt");
  std::string line, goldAmountStr;
  while (std::getline(inFile, line)) {
    if (line == std::to_string(hashedInput)) {
      std::getline(inFile, goldAmountStr);
      outFile << line << '\n' << newGold << '\n'; 
    } else {
      outFile << line << '\n';
    }
  }
  inFile.close();
  outFile.close();
  std::remove("accounts.txt");
  std::rename("temp.txt", "accounts.txt");
  return newGold;
}

void mainMenu() {
  gold = findGold();
  int ne;
  std::cin >> ne;
  gold = updateGold(ne);
  std::cout << "you have " << gold << " gold";
}

I was researching on this and all it said was to make sure that I have included the header file in both the main.cpp and functions.cpp file.

I have done that but I still get the huge linker error.

g++ main.cpp Functions.hpp Functions.cpp

/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0xc0): multiple definition of `SignUpUsername[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0xc0): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0xa0): multiple definition of `SignUpPassword[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0xa0): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x80): multiple definition of `Entered_username[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0x80): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x60): multiple definition of `Entered_password[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0x60): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x0): multiple definition of `hashedInput'; /tmp/ccpCKPCg.o:(.bss+0x0): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x14): multiple definition of `gold'; /tmp/ccpCKPCg.o:(.bss+0x14): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x40): multiple definition of `Entered_login[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0x40): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x20): multiple definition of `signuptrue[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0x20): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x8): multiple definition of `hasher[abi:cxx11]'; /tmp/ccpCKPCg.o:(.bss+0x8): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0xc): multiple definition of `action'; /tmp/ccpCKPCg.o:(.bss+0xc): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x10): multiple definition of `newgold'; /tmp/ccpCKPCg.o:(.bss+0x10): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x18): multiple definition of `IsSignUp'; /tmp/ccpCKPCg.o:(.bss+0x18): first defined here
/nix/store/039g378vc3pc3dvi9dzdlrd0i4q93qwf-binutils-2.39/bin/ld: /tmp/ccA63Jrg.o:(.bss+0x19): multiple definition of `goodLogin'; /tmp/ccpCKPCg.o:(.bss+0x19): first defined here
collect2: error: ld returned 1 exit status