Currently if you cancel a fulfillment that already is marked "status": "cancelled" you will receive a 422 HTTP Error response with the message "errors": "An error occurred, please try again". We’re shipping a change so that this action will instead return an HTTP 200 response. This will bring fulfillments in line with the orders endpoint where attempting to cancel an already cancelled order will return an HTTP 200 response.
As always feel free to post any comments or concerns in the thread below and happy coding.
Shopify Apps Team