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.