Sometimes "have" is the both helping verb and the main verb in the Past Perfect tense: I had already had breakfast before they arrived.

The reason it is confusing is that "had" is being used in two different ways. The first "had" is like "haber" in Spanish, the second is like "tener." Both "haber" and "tener" are "to have" in English. But the structure is just the same, they work together the same way. I hope my Spanish is correct here, I'm only intermediate in Spanish. I think "I had had a good day" = "Yo había tenido un buen día" and "I have had a good day" = "Yo he tenido un buen día."

Have is one of those auxiliary verbs that can also be used as an ordinary (main) verb. The present perfect form of have is have had. The past perfect form of have is had had (had + past participle form of have). The past perfect tense is used when we are talking about the past and want to refer back to an earlier past time. E.g. She felt marvelous after she had had a good night’s sleep. Key: 1st 'had' is the auxiliary, 2nd 'had' is the main verb. _________ I hope that helped! Feel free to checkout the great reviews from my wonderful students. Hope to hear from you soon! 😄👍📚🎯