Loss of nomination form by a private sector bank

Hello, My mother passed away and have a few fixed deposits. On claiming, bank informed me that on most of them, I am the nominee and rest, another name is mentioned as nominee. Now, there is no one who is known to me or my entire family, or even to my (deceased) mother, by that name, so definitely, it's some error done. Further, on the bank nomination register, there is only the first name of this nominee written and on the computer, they have included my surname. When requesting for the original nomination form, bank informs me that it has been lost. Further, they even go to the extent of saying that I am like a third party. I believe, that nomination form is supposed to be maintained by the bank forever and produced when demanded, by heir. I am requested instead to produce a succession certificate, when the mistake has been on their part. I even have a family tree provided by notary. Please let me know the recourse I can take in this matter without getting into the rigmarole of a succession certificate.