Notice from DDA to vacate flat

My father had purchased a DDA flat from the original allottee in 1992. The flat was allotted in 1991. In 2017, when we applied for freehold, DDA informed that the flat has not been given possession by them to the first allottee as some payment was pending. DDA had issued notice to my father in 2017 for vacating the flat. We have given a reply to DDA that we are willing to pay the dues and the flat may be regularised in my father's name. From 2017 till date, still some file work is going on in DDA regarding our flat, recently they called my father for verification of the original papers. They say that the original possession papers with my father issued by DDA in !991 is fraud. My contention is that can DDA claim that they have not given possession of the flat after 25 years and that too when it came to their notice when we applied for freehold. In all these years we had not received a single notice from DDA to pay the pending dues or to vacate the flat. The notice received from DDA in 2017 was the first and last notice.