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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 standard English, you is both singular and plural; it always takes a verb form that originally marked the word as plural, (i.e. you are, in common with we are and they are). This was not always so.