If you add to object component like this:
[RequireComponent(typeof(B))]
public class A : B {}

Component B will be added to the object.
Object should have only A.
It is not consistant with behavior in similar situations:
a) when A requeres A (then there will be only one A component)
b) when A requeres some other component B1, but object already has component A1 (where A1 inherits B1), B1 will not be added.

After you add A to an empty object, B will be added.
If you try to delete B you'll get an error "A depends on B". So you should delete A first and then B.

But if you add another A, you'll get one component B and two A. Then you can delete B. And after that you can delete one A. Last A cannot be deleted because of the bug 568895