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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 .
In Middle English, spoken from around 1150 to about 1500, the phrase was al be, which means although it may be. At some stage these words were all glued together as albeit, with the same meaning.