Selling Generic (unbranded) item

Hi There we sell items which are sold by other seller . The other seller has listed them as Generic/Generic1. We have uses their ASIN to avoid a duplicate listing, however the other seller is complaining that i am using his EAN/UPC as it was purchased by them and asking to remove the listing. But my understanding is that we used the amazon ASIN to sell the same. can someone explain will this affect my listing? if so what the other options for me as we have some FBA inventory .
What the things i should ask the seller to prove really what they are claiming?
appreciate your help and understanding