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I had had too much to drink last night when I attempted to drive home.
Yes, the first "had" is an auxiliary, or helping verb, and the second "had" is the main verb. "had had" is the Past Perfect Tense, which means that the action has already occurred.
For example, consider the following sentence: "Because SHE'D HAD so much to drink, she needed to take a taxi home." This sentence (without the use of the contraction) is actually: "Because she HAD HAD so much to drink, she needed to take a taxi home."
In other words, because "had had" tends to sound strange, we often abbreviate it in our speech by using contractions like "she'd had," "he'd had," or "I'd had." This way, it doesn't sound so odd. But, in formal writing (where you are not supposed to use contractions), you will see the full word written out....hence the "had had."
