1. Create object.
2. Add Rigidbody2D to the object
3. Create script like this:
[RequireComponent(typeof(Rigidbody))]
public class A : MonoBehaviour {}
4. Add it to the object.

You'll get error
"Can't add component 'Rigidbody' to Sphere because it conflicts with the existing 'Rigidbody2D' derived component!"
But component A will be still added to the object
You can do the same from a script. You can see it using AddA.cs script