can anybody explain it,why we use '"Have had" and "Had had" in a sentence?

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While “had had” can be used grammatically correctly, there’s really no real functional purpose in the second “had” and I wouldn’t bother using it.

“Have had” on the other hand, changes the meaning of the sentence because it turns “have” into the present perfect tense.

I have a lot of work to do = there is work yet to be done.

I had a lot of work to do = the work has already been completed.

I have had a lot of work to do (so far) = implies that while some work has already been completed, there is still more to come.