sub class constructor calling its "super constructor"

[oontvoo]

I have a super class and a sub class defined as follow

class Super
{
public:
	double x;


	Super ( double x ): x(x)
	{}

	Super()
	{
		Super(0);
	}

};

class Sub : public Super
{
public:
	double y;


	Sub ( double x, double y) : 	Super(x), y(y)
	{
	
	}
};

When I instantiate the Sub class

Sub mySub(1,2);

cout << mySub.x << endl << mySub.y << endl;

I'd get

1
2

as the output, which is obvious!
The thing is, when I tried to modify the Sub's constructor like this

Sub ( double x, double y )
{
     Super(x);
     this->y = y;
}

I got an error that read "error C2082: redefinition of formal parameter 'x'"

So! Can anyone explain what that means? And why there's a difference between two way of defining the constructor.
Thanks!

[jsmith]

Base class constructors can only be called via initializer list.