Thanks Lars for your answer!
Please go to inventory->configuration->operations types and set the filter "archived" and you will find the archived subcontracting operation. Unarchive this and open it in the inventory dashboard and you will get more details of what happens " behind" the receipt and register consumption than you mentioned in the subcontracting process.
For me there is a BUG that Odoo should fix because you can enter lots two times for the same operation.

I modified my answer, since I now understand what you ment by 'unarchive'. That archived operation is a manufacturing operation. I think it is not needed in the process. Perhaps in earlier version. (I checked for V14).
What is described in the documentation is that you register consumption when you do goods receipt. And with lot/ serial numbers.

"Traceability

In case the products received from the subcontractor contain tracked components, their serial or lot numbers need to be specified during the receipt.

"On the receipt of the subcontracted product, a Record Components button appears when necessary. Click on it to open a dialog and record the serial/lot numbers of the components. If the finished product is also tracked, its serial/lot number can be registered here too."

I am not surprised that you can enter the same lot number several times. There is really no check in Odoo for that. But for what operation do you refer to? Supply to the subcontracor or receive. Lot number for components or lot number for what you receive?