1) You won't get Duplicate Marriage Certificate but will be able to get certified copy of the copy of the original which is in the record of the Marriage officer/registrar. Now you may call it duplicate or not it hardly matters but its validity for any & all purpose is same as the original certificate. The words duplicate may not appear on the top of the marriage certificate now issued to you but it will be confirmed & certified as true copy of the original so go & get it & use it for any purpose you want.
2) Better way is go to Kolkata your jurisdiction registrar office...contact any staff inside who will help you to get the certified copy of the marriage certificate..since you have photo copy of marriage certificate know the date , it will be bit more easy for the staff to search.
3) There is procedure for obtain duplicate but the certified copy of the lost document clearly shows the status of marriage special registrar marriage act helps to found the number on process to receive search fees there after gave a complaint to the local police station which the document is misplaced from your custody the police after verification issued a certificate stating that the original was not traced the complaint ,certified copy ,paper publication to the public for a request to handover these 3 documants after the police issued not tracable there after the concerned officer issued certificate there is an endorsement on the foot of it as "DUPLICATE".