Stamp Duty Paid on Agreement for Sale But Not Registered in 1993 for Mumbai Flat

Scenario: (Mumbai Co-op Housing Society in process of verifying documents for purpose of conveyance of title from builder after 40 years, due to future redevelopment plans) 1. First owner of Flat entered into an Agreement of Sale with builder in 1987 2. First owner sold flat to myself in 1994. Agreement of Sale was executed in 1993, stamp duty paid, but not registered. 3. Second owner then signed all necessary forms addressed to Housing Society confirming receipt of funds, and transferring all rights, title and interest to myself. 4. Society transferred share certificate in my name since 1993 5. All society bills and municipal property taxes for that flat appear in my name since 1993 and have been paid 6. Society is now progressing toward deemed conveyance, and is requesting registered agreements for sale. Questions: 1. Since i paid stamp duty but did not register the Agreement for Sale document in 1993, what can i do now to rectify, and what will be the approximate cost . Note: this is housing society in Mumbai ? 2. Will non registration of the Agreement of Sale hamper the conveyance efforts of the society ? 3. What do I need to be aware of when selling the flat? Thank You