Nokia 1650
Battery Duplicate in Seal Pack

I have purchased a Nokia 1650 Mobile sets from Cellular Shoppe situated at 19, Arbinda Sarani, Kolkata - 700 005 against Invoice No. May/199 dated 20/05/09, after 2 days the battery of sets is not well Then I reached the shop keeper about that, the shoop keeper said The battery is Duplicate What should do about That Please suggest me