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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 .
The last sentence does not seem like a proper sentence to me; instead, the joining together of dependent clauses with therefore as the link.. The last part the word âthereforeâ use as a adverb. It means it can be interchangeable with words such as âsubsequentlyâ or âaccordinglyâ. That is why the last sentence is correct. In the last part the word âthereforeâ was used as an adverb.