Programming in D p288. Destructor called too many times.

[Brother Bill via Digitalmars-d-learn]

The destructor runs too many times.  I would expect it to run 
just once.
What am I doing "wrong"?

source/app.d
```
import std.stdio : writeln;
import std.string : format;
void main()
{
        bool aCondition = true;

        if (aCondition)
        {
                auto duration = Duration(7, 30, 15);
                writeln("Duration is: ", duration);
	}	// ← The destructor is executed for 'duration' at this point 
as duration goes out of scope.
}

struct Duration
{
        int hours;
        int minutes;
        int seconds;

        this(in int h, in int m, in int s)
        {
                writeln("Creating Duration: ", h, ":", m, ":", s);
                hours   = h;
                minutes = m;
                seconds = s;
        }

        ~this()
        {
		writeln("Destroying Duration: ", hours, ":", minutes, ":", 
seconds);
        }

        string toString() const
        {
                return format("%02d:%02d:%02d", hours, minutes, seconds);
        }
}
```

Console output:
```
Creating Duration: 7:30:15
Duration is: 07:30:15Destroying Duration: 7:30:15
Destroying Duration: 7:30:15

Destroying Duration: 7:30:15
Destroying Duration: 7:30:15
Destroying Duration: 7:30:15
```