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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 .
As as a prepositionWe use as with a noun to refer to the role or purpose of a person or thing: â¦. As as a conjunctionThe conjunction as has several different meanings. We use as when one event happens while another is in progress (âduring the time thatâ). In this case the verb after is often in the continuous form: â¦. The same asWe use as with the same to talk about identical things: â¦. As: simultaneous changesWe use as to introduce two events happening at the same time. After as with this meaning, we usually use a simple (rather than continuous) form of the verb: â¦.