1. Title of property does not exist in absence of registering the title deed,
2. In your case the stamp duty was paid but the sale deed was not registered by the 2nd owner,
3. Understand the 1st owner also did not register his title deed in absence of which he can not register the sale deed to the 2nd owner. This is the reason for which the 2nd owner could not register the sale deed,
4. First find out in whose name the said flat stands in the Registry office,
5. The person from whom you are buying the said flat should be shown as the title holder/owner of the said flat in the registration office,
6. Unless the above condition is fulfilled, you will not get title of the flat if you pay its consideration to the 2nd owner.