Unity3D: Snippet that prevents adding a component multiple times to a GameObject in the Unity Editor.

SingleComponentCheck.cs

#if UNITY_EDITOR
    void SingleComponentCheck() {
        var components = gameObject.GetComponents(this.GetType());

        foreach (var component in components) {
            if (component == this) continue;

            UnityEditor.EditorUtility.DisplayDialog("Can't add the same component multiple times!",
                string.Format("The component {0} can't be added because {1} already contains the same component.", this.GetType(), gameObject.name),
                "Cancel");

            DestroyImmediate(this);
        }
    }

    protected virtual void Reset() {
        Invoke("SingleComponentCheck",0);
    }
#endif