How can I call a editor script on scene load?

How can I call a editor script when the unity editor loads a scene?
Some kind of OnAwake equivalent.

class SaveManagerEditor extends Editor
{
	function DoOnSceneLoad ()
   	{
   		Debug.Log("SAVING");
   		CheckDuplicates();
   	}
}

Yes you can call it from the target script.
You would have to make the function public in order to access it from an external script.
If you don’t use instances but rather a global class, you should make the class or the function static.

public class SaveManagerEditor extends Editor
{
    public static function DoOnSceneLoad ()
    {
        Debug.Log("SAVING");
        CheckDuplicates();
    }
}

With this code you could call the function like this:

SaveManagerEditor.DoOnSceneLoad();

Be aware that editor classes don’t get compiled when you build your game.
So when you would use the code line above, you would get an error, as the editor class is unknown.
To prevent that, you can use directives (C#-Präprozessoranweisungen | Microsoft Learn).

#if UNITY_EDITOR
SaveManagerEditor.DoOnSceneLoad();
#endif

You can find all Unity releated directives here:

Edit:
I misunderstood the question and had to reedit the answer.

Greetings
Chillersanim