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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 .
We use has or have with a past participle to describe an action that started in the past and is (or may be) still going on. This construction is called the present-perfect. The past participle form of a regular verb is identical to the past form: it always ends in-ed :