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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 .
If you are unsure which to use, use who. Firstly, it is much more common than whom. Secondly, the use of whom is considered by many to be on its last legs in English. (Of course, Grammar Monster does not condone this practice, but if you don't have time to learn the difference, this advice will do for now.