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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 .
1 (grammar) A word used to join other words or phrases together into sentences. The specific conjunction used shows how the two joined parts are related. Example: Bread, butter and cheese. ( astronomy) The alignment of two bodies in the solar system such that they have the same longitude when seen from Earth.