why we use "have had" together?

"Have had" is using the verb have in the present perfect tense.

Consider the present tense sentence:

I have a lot of homework.

This means that I have a lot of homework now.

On the other hand, we use the present perfect tense to describe an event from the past that has some connection to the present. Compare the following two sentences:

• I had a lot of homework this week.
• I have had a lot of homework this week.

If I only say had, this means that "having a lot of homework this week" is a completed event, either because there is no expectation of more homework, or because the week is over.

If I say "have had", I connect the event to the present, so it is possible that I might have more homework, and I could say something like this on, e.g., a Wednesday (in the middle of the week).

Another example will illustrate the importance of the connection to now:

• I had a lot of homework last year.
• *I have had a lot of homework last year. (this sentence is bad!)

In the first sentence here, using had, the sentence is fine. But using "have had", the sentence is ungrammatical, because "last year" is always a completed event that is not connected to the present. But, as we know, the present perfect tense means that there is a connection to the present. So, the sentence sounds wrong, because the verb and the time are contradicting each other.