Multiple registration from the property Owner

Hi Sir, I(X) have purchased a property in Bangalore in year 2006 and registered on 30-OCT-2006 we have purchased from (Y) and made full settlement and transferred Khata and got electric meter and Paid tax upto date in my name, I have built a small house of 1BHK and gave it on rent in 2013 and already there is family Living there. We have purchased from (Y), Which they have purchased in 2004 with registered sale deed from (Z) GPA Holder (Not-Registered) just done a Notary in the year of 1995 from the Landlord with the full amount and Landlord has given in writing that i have taken a money i do not have any rights while giving it to GPA for (Z). Before selling it the (Z) GPA Holder and taken money in 1995, Landlord have also given GPA to another Person (W) in 1990 which is also not registered GPA just Notary with Local Advocate. The GPA Holder (W) has sold this land to other(A) party through Sale deed in 10-MAR-2006. Already we are in the property from 2006 till date no have come to us, I week ago we have received a call saying this property belongs to Party (A) and they have not even seen the property till now just came and asked for Site number and Survey number after that started claiming that they have own this property. As per EC Encumbrance certificate we have both the names Party (X) ME and Party (A) who have Claimed after 11 years. Landlord Died in 2013. My Question is Legally who is the owner of this property (X) or (A)? Both the GPA Holders have not registered GPA Holders. I have all the Legal documents, Khatta, Electricity Bill in my name what should be the next action. Please suggest me as soon as possible.