Dear Aspirant
You don't neet to register or pay again for round-2 as you have already done in round-1 for online all india quota or deemed/central universities UG counselling 2019 but you have to make fresh choices for round-2 as the choices submitted in round-1 will be treated as null and void and you have to use the same login credentials as used in round-1.
The Choice Filling has been started from 12th July, 2019 (3 pm) and this facility will continue till 14th July, 2019 (From 10:00 AM to 05:00 PM) and on 14th July you can lock the choices.
Before going for further steps, kindly read the instruction carefully given in links below
Https://medicalcounseling.nic.in/UGCounselling/Documents/SchemeUG2019.pdf
Https://medicalcounseling.nic.in/UGCounselling/Documents/FAQMCCUG2019.pdf
Hope this helps you.