Allotment letter

You have three options, which can be availed singularly and simultanously as well

1. File a suit for specific performance of contract before jurisdictional civil court.
2. Lodge a police or court complaint  for committing cheating and fraud.
3. Move to jurisdictional consumer disputes redressal commission for deficiency of service and unfair trade practice.

Thank you sir

Just wanted to check if I can file a case in RERA too?

Yes, you can file a case before jurisdictional RERA also.

Option No.2 of Dr. J. C. Vashista would be most appropriate. This is real cheating, selling the same property twice.    The new purchasers action is also dubious. He could have been duped or there was his connivance also.  I do not know which will give quick result. Converting a real criminal case into a civil case can help the offenders and prolong remedy.