Had Had
It's the longest grammatically correct contraction. It means without me, you couldn't have done it. Like when you fail a test and your friend takes it and passes.
'Without me, you couldn't have done it.' 'You failed, I passed, so I'm the hero.'
'Had had is the longest contraction ever.' 'It's like a sentence that took the whole day to say.'
'You had had the chance to do it yourself.' 'But you didn't, so I did it for you.'