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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 However, only mine and thine are used as possessive pronouns, as in it is thine and they were mine (not *they were my). ^  From the early Early Modern English period up until the 17th century, his was the possessive of the third person neuter it as well as of the third person masculine he.