If I was allotted a seat in first round of state quota and I join it but in second round of all India quota seat is also allotted I will
Hello there. Leaving any seat is never a big deal. Yes you can leave the seat. What you only need to do is visit the college where you have taken admission and write down application regarding cancellation of your admission and it's done.
Also you can apply for fees reimbursement through which you will get the fees paid by you back.