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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 .
it's me, and me's the first person singular, nominative case, agreeing with the verb it's, and governed by Squeers understood, as a acorn, a hour; but when the h is sounded, the a only is to be used, as a and, a art, a ighway,' replied Mr Squeers, quoting at random from the grammar.