Remove duplicate component

Questions & Answers
1

Unity has RequireComponent attribute to make sure the other component is there.
Also, this prevents the removal of the required component.

Would there be an attribute that prevents adding a component twice on an object?

I know I could do some kind of singleton process to remove a duplicate but in case there would be an easy way I am missing.

3

The attribute DisallowMultipleComponent was just added in 4.5:

Introduced DisallowMultipleComponentAttribute for disallowing multiple components of the same type being added to a single GameObject.

[DisallowMultipleComponent]
public class Script : MonoBehaviour {
    
}
2

You could do it in the Awake of the component that should only be added once:

  public class ThereCanBeOnlyOne<T> : MonoBehaviour {
       protected void Awake() {
             if(GetComponents<T>().Length > 1) {
                Destroy(this);
             }
       }
  }

  public class SomeComponent : ThereCanBeOnlyOne<SomeComponent> {
  }