Are you familiar with the three principal parts of a verb?
grow, grew, grown
talk, talked, talked
leave, left, left
sing, sang, sung
have, had, had
teach, taught, taught
Are you familiar with the formation of the perfect tenses, using the third of these principal parts with forms of have?
has grown, have grown, had grown
has left, have left, had left
has sung, have sung, had sung
The perfect tenses of the verb have are probably what you are seeing when you see have and had together. It's the same process shown above, but it seems confusing because the first form is used to make the perfect tense and the second is the third principal part of have.
has had, have had, had had
CJ