When you setup automatic consumption of project items on product receipt, but you've already consumed the item, the user gets an error message generated from the item requirement sales order: "Overdelivery of line is 100.00 percent, but the allowed overdelivery is only 0.00 percent.". I think the system should be a bit smarter than this, and recognize that the item requirement has already been consumed, and just present the user with an information message to that effect.

The scenario where we're getting this is a project where we're buying an outwork service item (e.g. transport services). Because the purchased item is a service item, we're not forcing the user to perform the product receipt before they post the item requirement packing slip - the service item allows negative inventory -  and the users are finding it quicker and easier to post the item requirement packing slip before they post the purchase order product receipt or invoice.