Builder defaults the loan and the lender issue notice under sarfasi 13/2

I have taken a flat in a high rise society in 2016, I took load from HDFC to pay to the builder. I have paid all amount to the builder and took the possession of the flat. After that I started following up with builder for registry, on which he always said due to some NOC it will take some time. Now in mid April 2018 we found that a housing bank put a notice stating that builder has taken some loan by mortgaging few flats in 2015 and since now builder stopped re-payment hence they issued the notice. Now the question is 1. As a buyer we had paid all the amount to the builder, so can the bank take the possession of my flat. 2. I had taken the home loan from HDFC in 2016 where as the property as per the notice already mortgaged in 2015. How come bank financed such property. 3. Its a fraud and clear cheating by the builder, what action we can take against him (he is already behind the bars because of multiple such cases). 4. What steps we need to take to safegurad our property as we already paid huge amount to builder and spend a lot of amount for internal work at the flat. Request for the expert suggestion to proceed further