Builder sold covered parking to two different customer

I have purchased resale flat but on sale deed & index2 covered car space number is mentioned, Builder has sold covered car parking to my seller first and later after 5 months builder sold same covered parking to other customer. But my seller come to know this fraud done by builder and soon he contacted builder. Builder has given allotment letter of open car parking on the date when society was already formed before. So second buyer was using parking till 15 years, In recent society AGM , society has removed my open parking space because allotment letter from builder is after society formation. Then i have taken possession of covered car parking because that parking was vacant till 4 months and i came to know that second buyer is reselling his flat. I talked with second buyer orally regarding parking dispute and he was agreed to give me parking after our formal communication. Now i am using covered car parking since 4 years, later his flat is sold to other customer. Other customer is not claiming that parking again because his owner was using parking for 15 years. Now he is saying that he has parking space number mentioned in his possession letter and occupation letter, i checked my document and i have found that parking space number is not mentioned in my possession letter and occupation letter. is possession letter and occupation letter given by builder is strong legal document to prove legal owner of parking ? OR Index 2 and sale deed is strong legal document which was registered in registry office. As per Index2 and sales deed, i am first buyer of parking.... Who is legal owner of parking after 15 years ? Please suggest