FDSGSG is absolutely correct. He should have immediately processed a refund on the $600 as soon as he saw the transaction. By not doing so he risked getting a chargeback and loosing not only the $600 but an additional $10 PayPal chargeback fee.

There is no risk of OP loosing $1,200 to two refunds if he went through the proper refund procedure. A transfer can only ever be refunded once.