Hello aspirant!
Hope you are doing well!

You haven't mentioned any state in specific, kindly do mention it.
The mop-up round criteria for AIQ NEET is
Eligibity criteria for mop-up round
1.Students who didn't get any seat in round 1st and 2nd counselling
2.Candidates who got admitted in round 1st and 2nd and secure the seat but didn't fill up status retention form.
If you get admission in second round state counselling, you can take part in mop up round counselling for upgrade only if you haven't filled retention form.

Also,
You should not have joined the college allotted in round 2 of counselling of NEET if you wish to participate in the mop-up round for Deemed/Central universities. Also you need to register yourself afresh for the mop-up round

But then these candidates will have their security amount forfeited.

For more refer to https://medicine.careers360.com/articles/neet-counselling-faqs

Hi.

I can only assume that you are referring to UP NEET Counselling. So, YES even if you have been allotted a seat in second round, you can participate in the MOP Round.

These are the steps you can follow for the Mop Round:

Step 1:-

Check the seat matrix available at different medical and dental colleges in order to secure the admission.

Step 2:-

Fill the online form on the official website of MCC and fill in your preference. We advise you to carefully fill in and select your preferences because they can not be changed once they have been filled.

Step 3:-

After selecting all the choices as per the merit list, the officials will declare the seat allotment result.

Step 4:-

Once the seats are allocated and the result is declared report for further formalities to the allocated institution.

Best of Luck