What does "duplicate symbol _heating_unit in BangBangControlTest.o and BangBangControl.o" mean?

Im receiving this error when trying to compile my code.

$ g++ -o BangBangControlTest BangBangControl.o BangBangControlTest.o ld: duplicate symbol _heating_unit in BangBangControlTest.o and BangBangControl.o for architecture x86_64 collect2: ld returned 1 exit status

I am new to C++ and can't find out what is wrong. I've searched through many tutorials and looked at similar error messages received by other stack users. Here are my classes. "BangBangControlTest.cpp"

// Test function
#include <iostream>
#include "BangBangControl.h"
using namespace std;

int main(){
  BangBangControl control(50, true, 75);

for(int i = 0; i < 50; i++){
std::cout << "Temp = " << control.update() << endl;
}

return 0;

}

"BangBangControl.cpp"

#include <iostream>
#include "BangBangControl.h"
using namespace std;

BangBangControl::BangBangControl(int temp, bool isOn, int initialTemp){
  heating_unit = HeatingUnit(isOn, initialTemp);
  temp_to_maintain = temp;
}

void BangBangControl::setTemp(int temp){temp_to_maintain = temp;}
int BangBangControl::getTemp(){return temp_to_maintain;}
int BangBangControl::update(){
  int b=heating_unit.tick();
  if (b  > temp_to_maintain + 2) heating_unit.turnOff();  if (b  < temp_to_maintain - 2) heating_unit.turnOn();
  return b;
}

"BangBangControl.h"

// BangBangControl header
#include <iostream>
#include "HeatingUnit.h"
using namespace std;

HeatingUnit heating_unit;
int temp_to_maintain;

class BangBangControl{

 public:
   BangBangControl(int, bool, int);
    void setTemp(int);
    int getTemp();
    int update();
 };

"HeatingUnit.cpp"

// HeatingUnit class implementation

#include <iostream>
#include "HeatingUnit.h"

using namespace std;

HeatingUnit::HeatingUnit(bool a, int b){
  isOn = a;
  temp = b;
}

void HeatingUnit::turnOn(){isOn = true;}

void HeatingUnit::turnOff(){isOn = false;}

int HeatingUnit::tick(){
  if(isOn && temp <= 100){
    return ++temp;

}
  else if((!isOn) && temp >= 0){
    return --temp;
  }
  else{
    return temp;
  }
}

"HeatingUnit.h"

#include <iostream>
using namespace std;

class HeatingUnit{
  public:
    bool isOn;
    int temp;

    HeatingUnit();
    HeatingUnit(bool, int);
    void turnOn();
    void turnOff();
    int tick();
};

You see that HeatingUnit heating_unit; in your header file? You need to put extern in front of it, and copy the original version without the extern to the .cpp file, optionally specifying an initial value there.

You can read more about this here: How do I use extern to share variables between source files?