query on input credit when moving from normal to composition scheme

as far as my understanding and what I've red in the provision of composition levy is - when normal tax payer opting to composition scheme - he shall pay an amount equal to the input credit on the stock lying on the immediate preceding switch over date.

why does tax normal tax payer has to pay amount equal to input tax again? As he should have paid the same to treat it as input.

does it not tantamount to double payment or what logic behind in collecting this amount?

pls correct if I'm wrong.

Regards,
Ranga