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So don't worry so much about how to use  had had  as a unit of grammar, they will come together naturally when you want to express the verb ' to have ' in the past perfect. Let's consider a different verb for a moment like  to want .
The abbreviation is probably what leads to the 'of' error. In the cases above, the word have is a helping verb; it helps to clarify the verb's tense. If I had known that you were coming, I would have baked a cake.. At that past time, I did not know that you were coming.