Why does C# support abstract overrides of abstract members?

Whilst browsing through some legacy code, I was surprised to encounter an abstract override of a member that was abstract by itself. Basically, something like this:

public abstract class A
{
    public abstract void DoStuff();    
}

public abstract class B : A
{
    public override abstract void DoStuff();  // <--- Why is this supported?
}

public abstract class C : B
{
    public override void DoStuff() => Console.WriteLine("!");    
}

Isn't the distinction between a virtual or abstract member always available to the compiler? Why does C# support this?

(This question is not a duplicate of What is the use of 'abstract override' in C#? because the DoStuff-method in class A is not virtual but abstract as well.)