Unauthorised transactions on credit card

I received email notification about a credit card swipe for a transaction worth 15000/- without my authorization. I replied immediately to freeze the card as I didnt make any transaction since the past 1.5 years on that card. However, later I realised that the email notification sent earlier by the bank had a different credit card number than mine and I mentioned the same to them. I kept on receiving statements from the bank for this incorrect credit card number. When I filed for disputed transactions they emailed me after 2-3 months with a transaction receipt containing MY CREDIT CARD number which is in contradiction with the statement. The transaction slip also had a signature which was not mine and I asked them for more proofs like CCTV footage or photos etc. which they have never shown to me or contacted to me after that. However now I am getting email from lawyers to pay credit card dues for the incorrect credit card number. How can I respond to this request? I will in no way pay for this as this is extremely unfair.