Pranav, According to the official information brochure https://view.mahacet.org/mahacet/admin/news_document/FinalNEETUG%202020%20Informationrochure04112020.pdf, the rules for mop up are as follows:

Candidates who are eligible for the Mop Up Round:

• Candidates who were allotted a seat and were admitted in Round 1 and Round 2 and secured the admission but did not fill the status retention form.
• Those Candidates who have been waitlisted and whose choice was not available during Round 1 and Round 2.

Candidates who are not eligible for the Mop Up Round:

• Candidates who have been allotted a seat but haven't joined.
• Candidates who have filled the Status retention form.
• Candidates  who joined and resigned from the seats allotted.

Now if you have been allotted a seat in Round 1 and the same seat was allotted again in Round 2, the question is did you fill the status retention form. If you did, then you are not eligible for the Mop Up Round. But if you did not fill the Status retention form and you meet the other eligibility criteria that have been mentioned here, then you are eligible for the mop up round.