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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 this issue we begin a three-part series on the use of who or whoever and whom or whomever in writing. Speakers of American English sometimes use who or whom interchangeably. However, in written English, distinctive forms still are observed.