Hello Erukulla,
According to the rules of All India Quota NEET counselling, you will not be allowed to attend mop up round counselling if you already have been alloted a seat in second round of counselling.
If you donot report to the alloted college of second round of counselling then you will be allowed to exit with forfeiture of security money (loss of security money) and if you have joined the college alloted in second round of counselling then you cannot resign from your seat.
If you wish to cancel your second round alloted seat, you should not report to college. In that case you will have to register fresh in mop up round but your security money of second round will not be refunded.
Hope this helps you.