Advocate Notice on Land Purchased

I have purchased a land from a Builder way back in 2012 along with registration and finally got the original Patta from the concern Tahasildar office. Post 6 years suddenly I have received one Advocate notice form the Son of the previous owner of the land mentioning that the builder has done some fraude activity during purchase of the said land in 2011/12 fro his father. When I informed the builder on this he had mentioned that land records are genuine and there was not anomaly happened that time. Need suggestion on this; 1. Is an advocate notice have any legal viability? 2. If the land is having litigation then how come the Tahasildar office had issued original Patta in favour of me which has been updated in the online portal too. 3. What should my stand now?