the same property register two times. which one is effective

for cancellation of document: Both the parties (buyer and seller) shall execute a cancellation deed for already executed the sale deed.   Otherwise any civil court or High Court or a State Government or Central Government is competent to cancel a registered sale deed.  

When the court has registered the land, it is presumed that court has followed laid down the law.  It prevails unless the other party prays for setting aside that court orders and cancellation of the sale deed executed by the court.

The facts, as stated, lacks clarity.  It is not understood how the Court has come into the transaction when the posting does not disclose of any court case.

1. No sale deed has been and can be executed /registered during pendency of balance of consideration as stated by you. The statement posted by you lacks clarity.

2. If the vendor (seller) has sold and registered the property during the term of agreement with your father, as stated by you,  he (your father) should move to the Court for specific performace of contract through a local prudent lawyer.

3. If you have already filed any case, what is the opinion and advise of the lawyer engaged /paid by you, who is well aware about facts and circumstances of the case, an able, competent and intelligent enough  to advise  and proceed???

If your father had entered into a registered sale agreement with the seller then he can file a suit for specific performance of contract seeking the seller to execute the registered sale deed in his favor after receiving the balance of sale consideration amount.

Your father should not file any suit for cancellation of the registered sale deed made by the seller in favor of another buyer.

It is not necessary, he need not be bothered about it since he is not a party to that sale deed and it is not binding on him.