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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 languages where the verb is inflected, it often agrees with its primary argument (the subject) in person, number or gender. With the exception of the verb to be, English shows distinctive agreements only in the third person singular, present tense form of verbs, which are marked by adding -s ( walks) or -es (fishes).