Stranger things have happened. A rejection letter followed by an appointment is not unheard of, but certainly rare. As recommended above, follow-up quickly. It's not too late to get ready for Beast.
A prank? Pretty elaborate and nasty if true. The envelope, letter, certificate, and postmark would take some time and research to pull off a convincing forgery.
It is most likely an administrative screw up in your MOC's office. Some MOC's send a congratulatory letter and a formal looking, but unofficial, certificate as a keepsake for an appointee - you won't be able to present it at R-Day and enter Beast. It is possible the MOC office inadvertently included your name on a list of intended recipients.