Company asking to pay for the product that i already returned?

Placed order-->Return the order-->Amount Refunded into my account-->Got call from company saying I didnt pay the amt but took the product and need to pay.I argued with them then they said will investigate.After few days got call from company saying by mistake they have refunded the amt into my account and asking me to refund. How I should go with this issues. I need to chk my payment with my bank. what would be the refund process?Even after refunding any issues will be thr.Bcoz other guy asked abt product. I didnt take i ask them to check in their CC camera that i left the product with tem.