No second instrument chargeable with duty shall be written upon a piece of stamped paper upon which an instrument chargeable with duty has already been written: 

Provided that nothing in this section shall prevent any endorsement which is duly stamped or is  not chargeable with duty being made upon any instrument for the  purpose of transferring any right  created or evidenced thereby,  or of acknowledging the receipt  of any money or goods the payment or delivery of which is secured thereby.1

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Footnote :

1. Endorsement on Mortgage documents are exempted from stamp duty vide this provision.