Good question. One I've answered before. Here is what I told them.
To understand “had had,” we need to take a look at the present perfect and past perfect tenses. Take this sentence: “I have had too many chocolates today.” That sentence is in the present perfect tense. You use that tense when you’re talking about a past action that is continuing into the present. This sentence means that I started eating chocolates in the past but the chocolate eating is continuing up to the present. Present perfect tense uses “has” and “have” plus the past participle, as in “have had” and “has gone.”
Now let’s put the chocolate sentence in the past tense. To do so, we’ll use past perfect tense, which uses “had” plus the past participle, as in “had had” and “had gone.” So in the sentence “I had had too many chocolates, so I was too full to eat dinner yesterday,” two things happened in the past. First was eating chocolates; second was trying to eat dinner.
When you have two past-tense occurrences, you use past perfect to express the action that came first. If you are using the verb “to have” in past perfect, you need to use two “had”s.
I hope this helps,