This error typically occurs when your POS pings a refund twice in a row, often without your realizing. 99% of the time, the refund has successfully processed -- this error almost serves as a confirmation that the refund has been processed, because it only shows when the refund is processed twice in a row.
However, if you have any concerns about whether this refund was successfully processed on RedShelf's side, please submit a request with our Support Team with a screenshot along with any order details available!