运算符重载赋值运算符重复释放内存

问题遇到的现象和发生背景

我在学习黑马的C++的重载运算符operator=时，按照它的要求

问题相关代码，请勿粘贴截图

#include <iostream>
using namespace std;
class Person
{
    friend ostream &operator<<(ostream &cout, Person p);

public:
    Person(int age, double height);
    ~Person();
    Person &operator=(Person &p);

private:
    int m_age;
    double m_height;
    int *m_grade;
};
Person::Person(int age, double height)
{
    m_age = age;
    m_height = height;
    m_grade = new int(100);
}
Person::~Person()
{
    if (m_grade != NULL)
    {
        delete m_grade;
        m_grade = NULL;
    }
}
ostream &operator<<(ostream &cout, Person p)
{
    cout << "age=" << p.m_age << "\t"
         << "height=" << p.m_height << "\tgrade=" << *p.m_grade;

    return cout;
}
Person &Person::operator=(Person &p)
{
    this->m_age = p.m_age;
    this->m_height = p.m_height;
    if (this->m_grade != NULL)
    {
        delete this->m_grade;
        this->m_grade = NULL;
    }
    this->m_grade = new int(*p.m_grade);
    return *this;
}
int main()
{
    Person p1(19, 179.8);
    Person p2(15, 170);
    p2 = p1;
    cout << p2 << endl;

    return 0;
}

运行结果及报错内容

在delete处崩溃，猜测是重复释放

我的解答思路和尝试过的方法

我认真考虑了是不是重复释放，可是我认为我重载没问题

我想要达到的结果

正常运行

friend ostream &operator<<(ostream &cout, Person &p); //Person &p，不用引用，这里调用一次析构函数


Person &Person::operator=(Person &p)
{
    this->m_age = p.m_age;
    this->m_height = p.m_height;
    if (this->m_grade != NULL) //没必要删了重新new
    {
        // delete this->m_grade;
        *(this->m_grade) = *p.m_grade;
    }
    else
        this->m_grade = new int(*p.m_grade);
    return *this;
}