I claimed my student loan interest cause I made payments and the tax form was in my name but now my cosigner wants to claim it

I understand they made also paid a voluntary payment even though they were not obligated to because I was not in default (Loan contract says they are not obligated to unless it goes in to default and any payments made prior to that are voluntary) ) problem is I already filed and just filed an amended tax return because I noticed an issue on a W2 i had to fix. I spoke with the loan company and I do have the right to claim it but what happens if my cosigner fights it?