Question:
The error message Serial Number, xxxx, already in the Inventory Stock Cost-Quantity File  occurs when inventory is serialized, and in Purchase Order, Enter Purchase Order's, when a Purchase Order with a negative quantity (return) is entered and saved. After the Purchase Order is received or received/invoiced, the error message will occur when the Purchase Order is posting.

Answer:
This is caused by an error in version 8.5

Resolution

Please update to version 8.5.01 or above.
The update may trigger a "win to win" conversation.