WHEN DO WE USE HAD HAD IN THE SAME SENTENCE ONE AFTER THE OTHER .???????????? plzzzzz answer!

Had had" indicates usage of past perfect tense in a sentence. It talks about an action completed before another past action. In the case of "had" it's normally a possession rather than an action (though "to have" can also imply an action).

Instances for your understanding are:

- I had had a surgery long time back.

 - I had had to let go of my favourite bike. 

Hope this explanation helps you!