if already a pg degree or diploma holder can that person join for another pg course with neet pg qualifying marks

If your question is if you can apply for another PG course through NEET PG, though you have already  PG, yes you can. As long as you meet the eligibility criteria, you can appear for NEET PG and if you qualify, you can attend the counselling and take admission. The basic eligibility criteria for NEET PG is that you should have passed MBBS and shpuld have completed the mandatory house surgeon. Assuming you have been able to complete a PG course earlier with the same qualification, there is no reason why you shouldnt join another PG course.