为什么这个构造函数被调用两次？

我有这个代码：

// Example program
#include <iostream>
#include <string>

class Hello{
    public:
    Hello(){std::cout<<"Hello world!"<<std::endl;}
};

class Base{
    public:
    Base(const Hello &hello){ this->hello = hello;}
    private:
    Hello hello;
};

class Derived : public Base{
    public:
    Derived(const Hello &hello) : Base(hello) {}
};

int main()
{
    Hello hello;
    Derived d(hello);
    return 0;
}

结果打印为：

Hello world!
Hello world!

为什么会发生这种情况？