Are virtual destructors inherited?

If I have a base class with a virtual destructor. Has a derived class to declare a virtual destructor too?

class base {
public:
    virtual ~base () {}
};

class derived : base {
public:
    virtual ~derived () {} // 1)
    ~derived () {}  // 2)
};

Concrete questions:

1. Is 1) and 2) the same? Is 2) automatically virtual because of its base or does it "stop" the virtualness?
2. Can the derived destructor be omitted if it has nothing to do?
3. What's the best practice for declaring the derived destructor? Declare it virtual, non-virtual or omit it if possible?