What does "That barcode is already used by another product" mean?

Each barcode value can belong to only one product in your store, so it can identify the right item when scanned. The value you typed is already saved on another product.

• Look for the value in the list on Plugins → Product Barcodes. Each row shows the ID of the product it belongs to.
• If it is on the wrong product, Delete that row first, then save it on the right one.
• If both products really are different, give one of them a new value. Leave Barcode value blank, or click Generate value.