1. You had purchased the 2nd parking slot from the owner of the flats before the Society was formed,
2. The Society's rule about the ownership of one parking slot for one flat owner can not be with enacted with retrospective effect,
3. Moreover, if the 2nd parking slot is not treated as your property then who will own it?
4. You have right on the said 2nd parking slot,
5. If you leave its possession to the society what will they do with it? Can they own it or sell it?
6. Society can not own it since it has not purchased it,
7. Send a legal notice to the society informing that you had purchased the said parking slot before the society was formed, so it can not be taken away from you,
8. If they still refuse to accept logic, file a declaratory suit before the Court praying for declaration that you are the title holder of the said 2nd parking slot.