grammaticality - Necessity of using "had had" when a single "had" could have sufficed?

If Sir John had had the slightest idea what was going to be the devastating effect of his son's handsome looks upon a certain beautiful courtesan and the tragedy which would ensue, he might have had second thought about continuing his journey without him. (Source: Vengeful Ghosts by Vida Derry)

I am having problem with this use of "had" (BOLD). How I understand using had is that it is used if some action (action 1) precedes another action (action 2) and "had" accompanies the first action (A perfect Past Perfect scenario). For example:

I had done it, before you did that.

Now in the highlighted text, Sir John did not have the slightest idea, he did not take his son with him and the devastating effects of his son's looks did happen. So how can "had" here be used when the action i.e. having a slightest idea has never happened to Sir John?

In my opinion, a single "had" (simple past) would have sufficed.

Answer

You are confused here by the fact that a past form may be used in two ways: as a marker of past tense and as a marker of counterfactuality. In this case, it must serve both uses, and it is that which requires the past perfect form.

Let's consider this in the present tense, and then 'backshift' it to the past. There are two constructions:

1. 
   

   
   
   

   Sir John is a judicious man. If he has the slightest idea of the result he may have second thoughts.

   
   

   
   

   This employs the simple present of HAVE and MAY to express a condition regarded as possible or plausible. Backshift that into the past—as reported speech, for instance—and you get this:

   
   

   
   

   Lord West told me that Sir John was a judicious man and if he had the slightest idea of the result he might have second thoughts.

   
   

   
   


2. 
   

   Note, however, that exactly the same constructions are used to express a present counterfactual:

   
   

   
   
   

   Sir John is an idiot. If he had the slightest idea of the result he might have second thoughts. But he has no such idea.

   
   

   
   

   The second sentence there has already used up all its available past forms to mark counterfactuality. To backshift it into the past you must add some sort of past marker. By convention, those markers are supplied by converting the past forms into past perfects:

   
   

   
   

   Lord West told me that Sir John was an idiot; if he had had the slightest idea of the result he might have had second thoughts. But he had no such idea.