If you are talking about PGIMER entrance exam, the page no# of the Prospectus states: Candidates who have already done, or are doing, MD/MS in any subject at the time of applying, shall not be considered for admission to MD/MS courses. If it is found, during active verification and at a later stage, that the candidate has given wrong information, his/her registration will be cancelled without any notice.

The entrance exam is for admission to MS/MD courses which are PG degree courses. And because you have already completed your PG, you are not eligible for this exam. This exam is for a course that you have already completed. So it is fair that you cannot apply.