Remove duplicate component

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.

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.

public class Script : MonoBehaviour {

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) {

  public class SomeComponent : ThereCanBeOnlyOne<SomeComponent> {