Double registration entry in Encumberance certificate

Double registration showing on Encumberance certificate. How to nullify the other entry legally? My father (Party B) bought a property from Party A in 1989 and got it registered. It seems the same person sold the property to Party C in 1992. Unfortunately, the property address was same as my father's but boundaries were are different. The party C then sold the property to Part D in 1998 with same boundaries as my fathers. This was brought to my fathers attention and it was amicably agreed by Party C to surrender the property after agreement with Party A. The Party C, Party D transaction was nullified in registration office. But Party A Party C entry was untouched. My father took the possession of the property in 1998 and constructed the house and have been living there from past 18 yrs. Now when we are planning to sell the Encumberance certificate shows the other entry. How can we get it nullified? What is the detailed legal procedure as Party A is no more alive? Party C might be alive but not aware of the address? Will Party C have rights on our property even though the boundaries in his registration are different? The buyers are afraid to buy the property as banks are not offering loan.