How to reset a component on duplication?

Late but for new readers: Here is the definitive way to do it!

 public class Foo : MonoBehaviour
 #if UNITY_EDITOR
     , ISerializationCallbackReceiver
 #endif
 {
     private GameObject childObject;
 
 #if UNITY_EDITOR
     [HideInInspector] [SerializeField] private GameObject _childObject;
     private bool isDuplicate = true;
 
     public void OnBeforeSerialize() 
     {
         // Security check in case of Reset.
         if(childObject)
         {
             // Serialize the childObject for the next time we duplicate.
             _childObject = childObject;
         } 
     }
 
     public void OnAfterDeserialize()
     {
         if(isDuplicate)
         {
             // Delay these calls since gameObjects can't be accessed during Deserialization.
             UnityEditor.EditorApplication.delayCall += OnDuplicate;
             UnityEditor.EditorApplication.delayCall += Reset;
         }

         // Security check in case of domain reload.
         if(_childObject)
         {
             // Reassign the serialized childObject.
             chilldObject = _childObject;
         }
     }
 
     private int siblingIndex = -1;
     private void OnDuplicate()
     {
         // Destroy the duplicated child.
         siblingIndex = childObject.transform.GetSiblingIndex();
         DestroyImmediate(transform.GetChild(siblingIndex).gameObject);
     }
 
     private void Reset()
     {
         // Create a new child and set it to our siblingIndex (in case of duplication).
         isDuplicate = false;
         childObject = new GameObject();
         childObject.transform.SetParent(transform);
         if(siblingIndex > -1)
         {
             childObject.transform.SetSiblingIndex(siblingIndex);
         }
     }
 #endif
 }

This way we can do whatever junk we need to do in OnDuplicate (if we need it) and still call Reset afterwards. Since this is all done with Serialization, we can keep our Awake method clean and wrap this all in an #if UNITY_EDITOR, no [ExecuteAlways] required!

If you do experience trouble with this please let me know.

You could save the "restored" version as a prefab, and instance off of that?

Hmm... Have you tried:

 [ExecuteInEditMode]
 public class myObject : MonoBehaviour
 {
     [NonSerialized] public bool isDuplicate = true;
 
     void Awake()
     {
         myObject mo = this as myObject;
         
         Debug.Log( "In Awake. isDuplicate: " + myObject.isDuplicate );
         
         if( myObject.isDuplicate )
             Reset();
     }
 
     void Reset()
     {
         myObject mo = this as myObject;
         
         // re-set/intialize NonSerialized members, shared meshes, etc...
         
         myObject.isDuplicate = false;
     }
 
 }

One drawback is, Reset() will get called 2 times when you Duplicate your object in the Unity editor. Sorry. :/

I wasn't able to find a good way to directly call the 'Reset' function that the Inspector menu has, but I was able to cheat by deleting/recreating the component when it's instance ID doesn't match the saved one:

 [ExecuteInEditMode]
 public class ResetTest : MonoBehaviour
 {
     public int TestNumber;
     [HideInInspector] public int MyID;
 
     private void OnEnable()
     {
         if (MyID == GetInstanceID()) return;
         if (MyID == 0) MyID = GetInstanceID();
         else
         {
             gameObject.AddComponent(GetType());
             DestroyImmediate(this);
         }
     }
 }

Down side is this will 'move' the component to the end of the component list on the duplicated object. If your component order is important, that would be a problem.

If you don't want to use the ever-finicky [ExecuteInEditMode], you can do the same trick from a custom editor:

 [CustomEditor(typeof (ResetTest))]
 public class ResetTestEditor : Editor
 {
     private void OnEnable()
     {
         var t = (ResetTest) target;
         if (t.MyID == t.GetInstanceID()) return;
 
         if (t.MyID == 0) t.MyID = t.GetInstanceID();
         else
         {
             t.gameObject.AddComponent(t.GetType());
             DestroyImmediate(t);
         }
     }
 }

Which also cleans up the the class:

 public class ResetTest : MonoBehaviour
 {
     public int TestNumber;
     [HideInInspector] public int MyID;
 }